As of December 31, 2013, we had $58.7 billion of cash, cash equivalents, and marketable securities. Cash equivalents and marketable securities are comprised of time deposits, money market and other funds, including cash collateral received related to our securities lending program, highly liquid debt instruments of the U.S. government and its agencies, debt instruments issued by foreign governments, debt instruments issued by municipalities in the U.S., corporate securities, mortgage-backed securities and asset-backed securities.
As of December 31, 2013, $33.6 billion of the $58.7 billion of cash, cash equivalents, and marketable securities was held by our foreign subsidiaries. If these funds are needed for our operations in the U.S., we would be required to accrue and pay U.S. taxes to repatriate these funds. However, our intent is to permanently reinvest these funds outside of the U.S. and our current plans do not demonstrate a need to repatriate them to fund our U.S. operations.
Our principal sources of liquidity are our cash, cash equivalents, and marketable securities, as well as the cash flow that we generate from our operations. At December 31, 2013, we had unused letters of credit for approximately $173 million. We believe that our sources of funding will be sufficient to satisfy our currently anticipated cash requirements through at least the next 12 months. Our liquidity could be negatively affected by a decrease in demand for our products and services. In addition, we may make acquisitions or license products and technologies complementary to our business and may need to raise additional capital through future debt or equity financing to provide for greater flexibility to fund any such acquisitions and licensing activities. Additional financing may not be available at all or on terms favorable to us.