Segment and Geographic Information The Companys operations consist of two operating segments, which are also its two reportable segments: the Graduate Program Segment and the Short Course Segment. The Companys Graduate Program Segment provides services to well-recognized nonprofit colleges and universities, primarily in the United States, to enable the online delivery of graduate programs. The Companys Short Course Segment provides premium online short courses to working professionals around the world through relationships with leading universities in the United States, the United Kingdom and South Africa. Graduate Program Segment For the year ended December 31, 2018 , three university clients each accounted for 10% or more of the Companys consolidated revenue, as follows: $86.9 million , $54.2 million and $42.7 million , which equaled 21% , 13% and 10% of the Companys consolidated revenue, respectively. For the year ended December 31, 2017 , four university clients each accounted for 10% or more of the Companys consolidated revenue, as follows: $77.6 million , $48.2 million , $30.1 million and $28.3 million , which equaled 27% , 17% , 11% and 10% of the Companys consolidated revenue, respectively. As of December 31, 2018 , two university clients each accounted for 10% or more of the Companys consolidated accounts receivable, net balance, as follows: $11.9 million and $11.8 million , which equaled 36% and 36% of the Companys consolidated accounts receivable, net balance, respectively. As of December 31, 2017 , two university clients each accounted for 10% or more of the Companys consolidated accounts receivable, net balance, as follows: $9.4 million and $2.0 million , which equaled 67% and 14% of the Companys consolidated accounts receivable, net balance, respectively. Short Course Segment For the year ended December 31, 2018 and 2017 , there were no customers or individual university clients that had revenue associated with it that accounted for 10% or more of the Companys consolidated revenue. In addition, as of December 31, 2018 and December 31, 2017 , no customers had accounts receivable, net balances that accounted for 10% or more of the Companys consolidated accounts receivable, net balance, as customers are individual students or third parties paying on their behalf, rather than university clients. For the year ended December 31, 2018 , offerings associated with three university clients each accounted for 10% or more of the segments revenue, and when combined, accounted for approximately 81% of the segments revenue. For the year ended December 31, 2017 , offerings associated with three university clients each accounted for 10% or more of the segments revenue, and when combined, accounted for approximately 82% of the segments revenue. Segment Performance The following table summarizes financial information regarding each reportable segments results of operations for the periods presented: Year Ended December 31, 2018 2017 2016 (in thousands) Revenue by segment* Graduate Program Segment $ 348,361 $ 270,432 $ 205,864 Short Course Segment 63,408 16,320  Total revenue $ 411,769 $ 286,752 $ 205,864 Segment profitability** Graduate Program Segment $ 16,839 $ 13,022 $ 4,541 Short Course Segment 816 (1,606 )  Total segment profitability $ 17,655 $ 11,416 $ 4,541 Segment profitability margin*** Graduate Program Segment 4.8 % 4.8 % 2.2 % Short Course Segment 1.3 (9.8 )  Total segment profitability margin 4.3 3.9 2.2 * The Company has excluded approximately $44,000 of intersegment revenues from the year ended December 31, 2018 . ** The Company defines segment profitability as net income or net loss, as applicable, before net interest income (expense), taxes, depreciation and amortization expense, foreign currency gains or losses, acquisition-related gains or losses and stock-based compensation expense. Some or all of these items may not be applicable in any given reporting period. *** The Company defines segment profitability margin as segment profitability as a percentage of the respective segments revenue. The following table reconciles net loss to total segment profitability: Year Ended December 31, 2018 2017 2016 (in thousands) Net loss $ (38,330 ) $ (29,423 ) $ (20,684 ) Adjustments: Interest income (5,173 ) (371 ) (383 ) Interest expense 108 87 35 Foreign currency loss 1,722 866  Depreciation and amortization expense 32,785 19,624 9,750 Income tax benefit (4,867 ) (1,297 )  Stock-based compensation expense 31,410 21,930 15,823 Total adjustments 55,985 40,839 25,225 Total segment profitability $ 17,655 $ 11,416 $ 4,541 The Companys total assets by segment are as follows: December 31, 2018 December 31, 2017 (in thousands) Total assets Graduate Program Segment $ 702,827 $ 359,597 Short Course Segment 104,527 122,465 Total assets $ 807,354 $ 482,062 Trade Accounts Receivable and Contract Liabilities The Companys trade accounts receivable and contract liabilities in each segment are as follows: December 31, 2018 December 31, 2017 (in thousands) Trade accounts receivable Graduate Program Segment accounts receivable, net of allowance for doubtful accounts of $0 for all periods presented $ 31,110 $ 12,520 Graduate Program Segment unbilled revenue 265 666 Short Course Segment accounts receivable, net of allowance for doubtful accounts of $257 and $287 as of December 31, 2018 and 2017, respectively 982 988 Total trade accounts receivable $ 32,357 $ 14,174 Contract liabilities Graduate Program Segment deferred revenue $ 2,864 $ 2,523 Short Course Segment deferred revenue 5,481 4,501 Total contract liabilities $ 8,345 $ 7,024 For the Graduate Program Segment, revenue recognized during the year ended December 31, 2018 that was included in the deferred revenue balance at the beginning of the year was $2.5 million . For the Short Course Segment, revenue recognized during the year ended December 31, 2018 that was included in the deferred revenue balance at the beginning of the year was $4.5 million . Contract Acquisition Costs The Graduate Program Segment had $0.3 million of net capitalized contract acquisition costs as of December 31, 2018 . For the year ended December 31, 2018 , the Company capitalized $0.3 million and recorded an immaterial amount of amortization expense in the Graduate Program Segment. Geographical Information The Companys non-U.S. revenue, which is based upon the currency of the country in which the university client primarily operates, was $33.9 million and $10.0 million for the years ended December 31, 2018 and 2017 , respectively, and was sourced entirely from the Short Course Segments operations outside of the U.S. The Companys long-lived tangible assets in non-U.S. countries as of December 31, 2018 and December 31, 2017 totaled approximately $1.2 million and $0.7 million , respectively.