Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 2,842,590	$ 2,434,145
Unrealized Gain	1,635	819
Unrealized Loss	(1,885)	(2,769)
Fair Value	2,842,340	2,432,195
Debt securities due in one year or less	1,574,317	1,388,537
Debt securities due after one year through three years	864,764	843,605
Total debt securities	2,439,081	2,232,142
Cash Equivalents [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	191,190	124,793
Unrealized Gain		1
Fair Value	191,190	124,794
Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	2,651,400	2,309,352
Unrealized Gain	1,635	818
Unrealized Loss	(1,885)	(2,769)
Fair Value	2,651,150	2,307,401
US Treasury Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	548,736	570,695
Unrealized Gain	91	253
Unrealized Loss	(817)	(635)
US Treasury Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	548,010	570,313
Foreign Government Debt Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	6,976	17,999
Unrealized Loss	(6)	(8)
Foreign Government Debt Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	6,970	17,991
Corporate Debt Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	1,883,688	1,645,468
Unrealized Gain	1,474	496
Unrealized Loss	(1,061)	(2,126)
Corporate Debt Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	1,884,101	1,643,838
Time Deposits
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	403,113	199,906
Unrealized Loss	(1)
Time Deposits | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	403,112	199,906
Equity Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	77	77
Unrealized Gain	70	70
Equity Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	$ 147	$ 147