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Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 2,043,996	$ 1,700,666
Unrealized Gain	245	441
Unrealized Loss	(2,474)	(845)
Debt securities due in one year or less	1,137,825	872,872
Due after one year through three years	828,848	763,003
Total debt securities	$ 1,966,673	$ 1,635,875
Loss on investments			$ (1,575)
Realized investment gains losses, description	Realized gains and losses on sales of investments were not material in 2015, 2014 and 2013.	Realized gains and losses on sales of investments were not material in 2015, 2014 and 2013.	Realized gains and losses on sales of investments were not material in 2015, 2014 and 2013.
Cash Equivalents [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 130,169	$ 67,051
Fair Value	130,169	67,051
Investments [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	1,913,827	1,633,615
Unrealized Gain	245	441
Unrealized Loss	(2,474)	(845)
Fair Value	1,911,598	1,633,211
Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	2,041,767	1,700,262
US Treasury Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	628,358	626,683
Unrealized Gain	16	246
Unrealized Loss	(1,218)	(157)
US Treasury Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	627,156	626,772
Foreign Government Debt Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	15,216	24,998
Unrealized Loss	(17)
Foreign Government Debt Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	15,199	24,998
Corporate Debt Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	1,325,398	984,668
Unrealized Gain	159	125
Unrealized Loss	(1,239)	(688)
Corporate Debt Securities | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	1,324,318	984,105
Time Deposits
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	74,947	64,240
Time Deposits | Fair Value Measurements, Recurring
Schedule of Available-for-Sale Securities [Line Items]
Fair Value	74,947	64,240
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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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