revenues from customers existing at or prior to december 31 , 2016 grew by $ 21.5 million to $ 219.4 million during 2017 . subscriptions from new customers added in 2017 contributed $ 11.4 million to the increase in revenues . we expect revenue growth from existing and new customers to continue . we market and sell our solutions to enterprises , government entities and small and medium-sized businesses across a broad range of industries , including education , financial services , government , healthcare , insurance , manufacturing , media , retail , technology and utilities . as of december 31 , 2017 , we had over 10,300 customers in more than 130 countries , including a majority of each of the forbes global 100 and fortune 100. in 2017 , 2016 and 2015 , approximately 70 % , 71 % and 70 % , respectively , of our revenues were derived from customers in the united states . we sell our solutions to enterprises and government entities primarily through our field sales force and to small and medium-sized businesses through our inside sales force . we generate a significant portion of sales through our channel partners , including managed service providers , value-added resellers and consulting firms in the united states and internationally . we have had continued revenue growth over the past three years . our revenues increased from $ 164.3 million in 2015 to $ 197.9 million in 2016 , and reached $ 230.8 million in 2017 , representing period-over-period increases of $ 33.6 million and $ 32.9 million , or 20 % and 17 % , respectively . we generated net income of $ 15.9 million in 2015 , $ 19.2 million in 2016 , and $ 40.4 million in 2017 . 41 key metric in addition to measures of financial performance presented in our consolidated financial statements , we monitor the key metric set forth below to help us evaluate growth trends , establish budgets , measure the effectiveness of our sales and marketing efforts and assess operational efficiencies . replace_table_token_4_th adjusted ebitda we monitor adjusted ebitda , a non-gaap financial measure , to analyze our financial results and believe that it is useful to investors , as a supplement to u.s. gaap measures , in evaluating our ongoing operational performance and enhancing an overall understanding of our past financial performance . we believe that adjusted ebitda helps illustrate underlying trends in our business that could otherwise be masked by the effect of the income or expenses that we exclude in adjusted ebitda . furthermore , we use this measure to establish budgets and operational goals for managing our business and evaluating our performance . we also believe that adjusted ebitda provides an additional tool for investors to use in comparing our recurring core business operating results over multiple periods with other companies in our industry . adjusted ebitda should not be considered in isolation from , or as a substitute for , financial information prepared in accordance with u.s. gaap . we calculate adjusted ebitda as net income before ( 1 ) other ( income ) expense , net , which includes interest income , interest expense and other income and expense , ( 2 ) provision for ( benefit from ) income taxes , ( 3 ) depreciation and amortization of property and equipment , ( 4 ) amortization of intangible assets , ( 5 ) stock-based compensation , ( 6 ) non-recurring expenses and ( 7 ) acquisition-related expenses that do not reflect ongoing costs of operating the business . the following unaudited table presents the reconciliation of net income to adjusted ebitda for each of the periods presented . replace_table_token_5_th ( 1 ) adjusted ebitda for 2017 excludes approximately $ 0.1 million of compensation expense from the acquisition of netwatcher . ( 2 ) adjusted ebitda for 2016 excludes approximately $ 0.7 million of a non-recurring expense related to the remittance of payroll taxes from fiscal year 2013 through may 2016. during this same period , we have not excluded any amounts related to other non-recurring items from adjusted ebitda because we have considered such amounts to be immaterial . 42 limitations of adjusted ebitda adjusted ebitda , a non-gaap financial measure , has limitations as an analytical tool , and should not be considered in isolation from or as a substitute for the measures presented in accordance with u.s. gaap . some of these limitations are : adjusted ebitda does not reflect certain cash and non-cash charges that are recurring ; adjusted ebitda does not reflect income tax payments that reduce cash available to us ; adjusted ebitda excludes depreciation and amortization of property and equipment and , although these are non-cash charges , the assets being depreciated and amortized may have to be replaced in the future ; and other companies , including companies in our industry , may calculate adjusted ebitda differently or not at all , which reduces its usefulness as a comparative measure . because of these limitations , adjusted ebitda should be considered alongside other financial performance measures , including revenues , net income , cash flows from operating activities and our financial results presented in accordance with u.s. gaap . 43 key components of results of operations revenues we derive revenues from the sale of subscriptions to our security and compliance solutions , which are delivered on our cloud platform . subscriptions to our solutions allow customers to access our cloud-based security and compliance solutions through a unified , web-based interface . customers generally enter into one year renewable subscriptions . the subscription fee entitles the customer to an unlimited number of scans for a specified number of devices or web applications and , if requested by a customer as part of their subscription , a specified number of physical or virtual scanner appliances . story_separator_special_tag in some limited cases , we also provide certain computer equipment used to extend our qualys cloud platform into our customers ' private cloud environment . customers are required to return physical scanner appliances and computer equipment if they do not renew their subscriptions . subscriptions for unlimited scans and certain limited scan arrangements with firm expiration dates are recognized ratably over the period in which the services are performed , generally one year . we recognize revenues for certain other limited scan arrangements , where expiration dates can be extended , on an as-used basis . we recognize the subscription of physical scanner appliances and other computer equipment as revenues ratably over the period of the subscription , which is commensurate with the term of the related subscription . because the customer 's access to our cloud solutions are delivered at the same time or within close proximity to the delivery of physical scanner appliances and the terms are commensurate for these services and equipment , we consider these elements as a single unit of accounting recognized ratably over the subscription term . physical equipment ( scanners and private cloud platforms ) are accounted for as operating leases and revenue is recognized over the subscription term . costs of shipping and handling charges associated with physical scanner appliances and other computer equipment are included in cost of revenues . deferred r evenues consist of revenues billed or received that will be recognized in the future under subscriptions existing at the balance sheet date . income taxes we are subject to income taxes in the united states as well as other tax jurisdictions in which we conduct business . earnings from our non-u.s. activities are subject to local income tax and may also be subject to u.s. income tax . 55 income tax expense or benefit is recognized for the amount of taxes payable or refundable for the current year , and for deferred tax assets and liabilities for the tax consequences of events that have been recognized in an entity 's financial statements or tax returns . we must make significant assumptions , judgments and estimates to determine our current provision for ( benefit from ) income taxes , our deferred tax assets and liabilities , and any valuation allowance to be recorded against our deferred tax assets . our judgments , assumptions and estimates relating to the current provision for ( benefit from ) income taxes include the geographic mix and amount of income ( loss ) , our interpretation of current tax laws , and possible outcomes of current and future audits conducted by foreign and domestic tax authorities . our judgments also include anticipating the tax positions we will record in the financial statements before actually preparing and filing the tax returns . our estimates and assumptions may differ from the actual results as reflected in our income tax returns and we record the required adjustments when they are identified or resolved . changes in our business , tax laws or our interpretation of tax laws , and developments in current and future tax audits , could significantly impact the amounts provided for income taxes in our results of operations , financial position , or cash flows . deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to tax benefit carry-forwards and to differences between the financial statement amounts of assets and liabilities and their respective tax basis . we regularly review our deferred tax assets for recoverability and establish a valuation allowance if it is more likely than not that some portion or all of the deferred tax assets will not be realized . to make this assessment , we take into account predictions of the amount and category of taxable income from various sources and all available positive and negative evidence about these possible sources of taxable income . the weight given to the potential effect of negative and positive evidence is commensurate with the extent to which the strength of the evidence can be objectively verified . based on the analysis of positive and negative factors noted above , we do not have a valuation allowance against u.s. federal and certain state deferred tax assets . we believe it is more likely than not that our california deferred tax assets will not be realized because the income attributed to california is not expected to be sufficient to recognize these deferred tax assets . accordingly , we continue to record a valuation allowance as of december 31 , 2017 for our california deferred tax assets . if , in the future , we determine that these deferred tax assets are more likely than not to be realized , a release of all or part , of the related valuation allowance could result in an income tax benefit in the period such determination is made . we recognize an income tax expense or benefit with respect to uncertain tax positions in our financial statements that we judge is more likely than not to be sustained solely on its technical merits in a tax audit , including resolution of any related appeals or litigation processes . to make this judgment , we must interpret complex and sometimes ambiguous tax laws , regulations and administrative practices . if an income tax position meets the more likely than not recognition threshold , then we must measure the amount of the tax benefit to be recognized by determining the largest amount of tax benefit that has a greater than a 50 % likelihood of being realized upon effective settlement with a taxing authority that has full knowledge of all of the relevant facts . it is inherently difficult and subjective to estimate such amounts , as this requires us to determine the probability of various possible settlement outcomes . to determine if a tax position is effectively settled after a tax examination has been completed , we must also estimate the likelihood that another

liquidity and capital resources at december 31 , 2017 , our principal source of liquidity was cash , cash equivalents and short-term and long-term investments of $ 355.6 million , including $ 7.5 million held outside of the united states by our foreign subsidiaries . we do not anticipate that we will need funds generated from foreign operations to fund our domestic operations . however , if we repatriate these funds , we could be subject to foreign withholding taxes . we have experienced positive cash flows from operations during the years ended december 31 , 2017 , 2016 and 2015 , respectively . we believe our existing cash , cash equivalents , short-term and long-term investments , and cash from operations will be sufficient to fund our operations for at least the next twelve months . we expect to spend approximately $ 23.0 to $ 28.0 million through december 31 , 2018 for capital expenditures , primarily related to infrastructure to support the anticipated growth in our business . our future capital requirements will depend on many factors , including our rate of revenue growth , the expansion of our sales and marketing activities , the timing , type and extent of our spending on research and development efforts , international expansion and investment in data centers . we may also seek to invest in or acquire complementary businesses or technologies . cash flows the following summary of cash flows for the periods indicated has been derived from our consolidated financial statements included elsewhere in this report : replace_table_token_21_th cash flows from operating activities in 2017 , cash provided by operating activities of $ 107.6 million was primarily due to $ 40.4 million of net income , as adjusted by increases in deferred revenues of $ 29.8 million , attributable to our continued growth in sales ; non-cash items including depreciation and amortization expense of $ 20.6 million and stock-based compensation expense of $ 27.0 million ; and an increase in other noncurrent liabilities of $ 7.3 million , primarily attributable to deferred rent relating to our office facility .


 Write a concise suspicious activity report of the following text delimited by triple backquotes. Return your response as a formal report which covers the key points highlighting the location,dates,entity names and account holder information in chronological order of the text.
     ```liquidity and capital resources at december 31 , 2017 , our principal source of liquidity was cash , cash equivalents and short-term and long-term investments of $ 355.6 million , including $ 7.5 million held outside of the united states by our foreign subsidiaries . we do not anticipate that we will need funds generated from foreign operations to fund our domestic operations . however , if we repatriate these funds , we could be subject to foreign withholding taxes . we have experienced positive cash flows from operations during the years ended december 31 , 2017 , 2016 and 2015 , respectively . we believe our existing cash , cash equivalents , short-term and long-term investments , and cash from operations will be sufficient to fund our operations for at least the next twelve months . we expect to spend approximately $ 23.0 to $ 28.0 million through december 31 , 2018 for capital expenditures , primarily related to infrastructure to support the anticipated growth in our business . our future capital requirements will depend on many factors , including our rate of revenue growth , the expansion of our sales and marketing activities , the timing , type and extent of our spending on research and development efforts , international expansion and investment in data centers . we may also seek to invest in or acquire complementary businesses or technologies . cash flows the following summary of cash flows for the periods indicated has been derived from our consolidated financial statements included elsewhere in this report : replace_table_token_21_th cash flows from operating activities in 2017 , cash provided by operating activities of $ 107.6 million was primarily due to $ 40.4 million of net income , as adjusted by increases in deferred revenues of $ 29.8 million , attributable to our continued growth in sales ; non-cash items including depreciation and amortization expense of $ 20.6 million and stock-based compensation expense of $ 27.0 million ; and an increase in other noncurrent liabilities of $ 7.3 million , primarily attributable to deferred rent relating to our office facility .
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 Suspicious Activity Report : revenues from customers existing at or prior to december 31 , 2016 grew by $ 21.5 million to $ 219.4 million during 2017 . subscriptions from new customers added in 2017 contributed $ 11.4 million to the increase in revenues . we expect revenue growth from existing and new customers to continue . we market and sell our solutions to enterprises , government entities and small and medium-sized businesses across a broad range of industries , including education , financial services , government , healthcare , insurance , manufacturing , media , retail , technology and utilities . as of december 31 , 2017 , we had over 10,300 customers in more than 130 countries , including a majority of each of the forbes global 100 and fortune 100. in 2017 , 2016 and 2015 , approximately 70 % , 71 % and 70 % , respectively , of our revenues were derived from customers in the united states . we sell our solutions to enterprises and government entities primarily through our field sales force and to small and medium-sized businesses through our inside sales force . we generate a significant portion of sales through our channel partners , including managed service providers , value-added resellers and consulting firms in the united states and internationally . we have had continued revenue growth over the past three years . our revenues increased from $ 164.3 million in 2015 to $ 197.9 million in 2016 , and reached $ 230.8 million in 2017 , representing period-over-period increases of $ 33.6 million and $ 32.9 million , or 20 % and 17 % , respectively . we generated net income of $ 15.9 million in 2015 , $ 19.2 million in 2016 , and $ 40.4 million in 2017 . 41 key metric in addition to measures of financial performance presented in our consolidated financial statements , we monitor the key metric set forth below to help us evaluate growth trends , establish budgets , measure the effectiveness of our sales and marketing efforts and assess operational efficiencies . replace_table_token_4_th adjusted ebitda we monitor adjusted ebitda , a non-gaap financial measure , to analyze our financial results and believe that it is useful to investors , as a supplement to u.s. gaap measures , in evaluating our ongoing operational performance and enhancing an overall understanding of our past financial performance . we believe that adjusted ebitda helps illustrate underlying trends in our business that could otherwise be masked by the effect of the income or expenses that we exclude in adjusted ebitda . furthermore , we use this measure to establish budgets and operational goals for managing our business and evaluating our performance . we also believe that adjusted ebitda provides an additional tool for investors to use in comparing our recurring core business operating results over multiple periods with other companies in our industry . adjusted ebitda should not be considered in isolation from , or as a substitute for , financial information prepared in accordance with u.s. gaap . we calculate adjusted ebitda as net income before ( 1 ) other ( income ) expense , net , which includes interest income , interest expense and other income and expense , ( 2 ) provision for ( benefit from ) income taxes , ( 3 ) depreciation and amortization of property and equipment , ( 4 ) amortization of intangible assets , ( 5 ) stock-based compensation , ( 6 ) non-recurring expenses and ( 7 ) acquisition-related expenses that do not reflect ongoing costs of operating the business . the following unaudited table presents the reconciliation of net income to adjusted ebitda for each of the periods presented . replace_table_token_5_th ( 1 ) adjusted ebitda for 2017 excludes approximately $ 0.1 million of compensation expense from the acquisition of netwatcher . ( 2 ) adjusted ebitda for 2016 excludes approximately $ 0.7 million of a non-recurring expense related to the remittance of payroll taxes from fiscal year 2013 through may 2016. during this same period , we have not excluded any amounts related to other non-recurring items from adjusted ebitda because we have considered such amounts to be immaterial . 42 limitations of adjusted ebitda adjusted ebitda , a non-gaap financial measure , has limitations as an analytical tool , and should not be considered in isolation from or as a substitute for the measures presented in accordance with u.s. gaap . some of these limitations are : adjusted ebitda does not reflect certain cash and non-cash charges that are recurring ; adjusted ebitda does not reflect income tax payments that reduce cash available to us ; adjusted ebitda excludes depreciation and amortization of property and equipment and , although these are non-cash charges , the assets being depreciated and amortized may have to be replaced in the future ; and other companies , including companies in our industry , may calculate adjusted ebitda differently or not at all , which reduces its usefulness as a comparative measure . because of these limitations , adjusted ebitda should be considered alongside other financial performance measures , including revenues , net income , cash flows from operating activities and our financial results presented in accordance with u.s. gaap . 43 key components of results of operations revenues we derive revenues from the sale of subscriptions to our security and compliance solutions , which are delivered on our cloud platform . subscriptions to our solutions allow customers to access our cloud-based security and compliance solutions through a unified , web-based interface . customers generally enter into one year renewable subscriptions . the subscription fee entitles the customer to an unlimited number of scans for a specified number of devices or web applications and , if requested by a customer as part of their subscription , a specified number of physical or virtual scanner appliances . story_separator_special_tag in some limited cases , we also provide certain computer equipment used to extend our qualys cloud platform into our customers ' private cloud environment . customers are required to return physical scanner appliances and computer equipment if they do not renew their subscriptions . subscriptions for unlimited scans and certain limited scan arrangements with firm expiration dates are recognized ratably over the period in which the services are performed , generally one year . we recognize revenues for certain other limited scan arrangements , where expiration dates can be extended , on an as-used basis . we recognize the subscription of physical scanner appliances and other computer equipment as revenues ratably over the period of the subscription , which is commensurate with the term of the related subscription . because the customer 's access to our cloud solutions are delivered at the same time or within close proximity to the delivery of physical scanner appliances and the terms are commensurate for these services and equipment , we consider these elements as a single unit of accounting recognized ratably over the subscription term . physical equipment ( scanners and private cloud platforms ) are accounted for as operating leases and revenue is recognized over the subscription term . costs of shipping and handling charges associated with physical scanner appliances and other computer equipment are included in cost of revenues . deferred r evenues consist of revenues billed or received that will be recognized in the future under subscriptions existing at the balance sheet date . income taxes we are subject to income taxes in the united states as well as other tax jurisdictions in which we conduct business . earnings from our non-u.s. activities are subject to local income tax and may also be subject to u.s. income tax . 55 income tax expense or benefit is recognized for the amount of taxes payable or refundable for the current year , and for deferred tax assets and liabilities for the tax consequences of events that have been recognized in an entity 's financial statements or tax returns . we must make significant assumptions , judgments and estimates to determine our current provision for ( benefit from ) income taxes , our deferred tax assets and liabilities , and any valuation allowance to be recorded against our deferred tax assets . our judgments , assumptions and estimates relating to the current provision for ( benefit from ) income taxes include the geographic mix and amount of income ( loss ) , our interpretation of current tax laws , and possible outcomes of current and future audits conducted by foreign and domestic tax authorities . our judgments also include anticipating the tax positions we will record in the financial statements before actually preparing and filing the tax returns . our estimates and assumptions may differ from the actual results as reflected in our income tax returns and we record the required adjustments when they are identified or resolved . changes in our business , tax laws or our interpretation of tax laws , and developments in current and future tax audits , could significantly impact the amounts provided for income taxes in our results of operations , financial position , or cash flows . deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to tax benefit carry-forwards and to differences between the financial statement amounts of assets and liabilities and their respective tax basis . we regularly review our deferred tax assets for recoverability and establish a valuation allowance if it is more likely than not that some portion or all of the deferred tax assets will not be realized . to make this assessment , we take into account predictions of the amount and category of taxable income from various sources and all available positive and negative evidence about these possible sources of taxable income . the weight given to the potential effect of negative and positive evidence is commensurate with the extent to which the strength of the evidence can be objectively verified . based on the analysis of positive and negative factors noted above , we do not have a valuation allowance against u.s. federal and certain state deferred tax assets . we believe it is more likely than not that our california deferred tax assets will not be realized because the income attributed to california is not expected to be sufficient to recognize these deferred tax assets . accordingly , we continue to record a valuation allowance as of december 31 , 2017 for our california deferred tax assets . if , in the future , we determine that these deferred tax assets are more likely than not to be realized , a release of all or part , of the related valuation allowance could result in an income tax benefit in the period such determination is made . we recognize an income tax expense or benefit with respect to uncertain tax positions in our financial statements that we judge is more likely than not to be sustained solely on its technical merits in a tax audit , including resolution of any related appeals or litigation processes . to make this judgment , we must interpret complex and sometimes ambiguous tax laws , regulations and administrative practices . if an income tax position meets the more likely than not recognition threshold , then we must measure the amount of the tax benefit to be recognized by determining the largest amount of tax benefit that has a greater than a 50 % likelihood of being realized upon effective settlement with a taxing authority that has full knowledge of all of the relevant facts . it is inherently difficult and subjective to estimate such amounts , as this requires us to determine the probability of various possible settlement outcomes . to determine if a tax position is effectively settled after a tax examination has been completed , we must also estimate the likelihood that another

