subsidiaries that have not been previously taxed since we intend to invest such undistributed earnings indefinitely outside of the U.S. If our intent changes or if these funds are needed for our U.S. operations, we would be required to accrue or pay U.S. taxes on some or all of these undistributed earnings and our effective tax rate would be adversely affected. We are also currently subject to audit in various jurisdictions, and these jurisdictions may assess additional income tax liabilities against us. Developments in an audit, litigation, or the relevant laws, regulations, administrative practices, principles, and interpretations could have a material effect on our operating results or cash flows in the period or periods for which that development occurs, as well as for prior and subsequent periods. For instance, the IRS is seeking to increase our U.S. taxable income related to transfer pricing with our foreign subsidiaries for transactions undertaken in 2005 and 2006, and we are currently contesting the matter in U.S. Tax Court. In addition to the risk of additional tax for 2005 and 2006 transactions, if this litigation is adversely determined or if the IRS were to seek transfer pricing adjustments of a similar nature for transactions in subsequent years, we could be subject to significant additional tax liabilities. In addition, in October 2014, the European Commission opened a formal investigation to examine whether decisions by the tax authorities in Luxembourg with regard to the corporate income tax paid by certain of our subsidiaries comply with European Union rules on state aid. If this matter is adversely resolved, Luxembourg may be required to assess, and we may be required to pay, additional amounts with respect to current and prior periods and our taxes in the future could increase. Although we believe our tax estimates are reasonable, the final outcome of tax audits, investigations, and any related litigation could be materially different from our historical income tax provisions and accruals. 

Recent Accounting Pronouncements 

See Item 8 of Part II, “Financial Statements and Supplementary Data—Note 1—Description of Business and Accounting Policies—Recent Accounting Pronouncements.” 

Liquidity and Capital Resources 

Our principal sources of liquidity are cash flows generated from operations and our cash, cash equivalents, and marketable securities balances, which, at fair value, were $19.8 billion , $17.4 billion , and $12.4 billion as of December 31, 2015 , 2014 , and 2013 . Amounts held in foreign currencies were $7.3 billion, $5.4 billion, and $5.6 billion as of December 31, 2015 , 2014 , and 2013 , and were primarily Euros, Japanese Yen, and British Pounds. 

Cash provided by operating activities was $11.9 billion , $6.8 billion , and $5.5 billion in 2015 , 2014 , and 2013 . Our operating cash flows result primarily from cash received from our consumer, seller, developer, enterprise, and content creator customers, advertising agreements, and our co-branded credit card agreements, offset by cash payments we make for products and services, employee compensation (less amounts capitalized related to internal-use software