Cash and Investments - Available-for-Sale Investments be Contractual Maturity Date (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Cash and Cash Equivalents [Abstract]
Amortized cost, due in one year or less	$ 1,653,937
Estimated fair market value, due in one year or less	1,652,844
Amortized cost, due after one year through two years	617,493
Estimated fair market value, due after one year through two years	615,492
Amortized cost, due after two years through three years	305,911
Estimated fair market value, due after two years through three years	304,809
Amortized cost, total	2,577,341
Estimated fair market value, total	$ 2,573,145