Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,247,634	$ 1,391,721
Gross Unrealized Gains	925	474
Gross Unrealized Losses	(1,204)	(2,561)
Aggregate Fair Value	1,247,355	1,389,634
Short-Term Marketable Securities	411,382	519,642
Long-Term Marketable Securities	835,973	869,992
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	68,800	0
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		39
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Aggregate Fair Value		39
Short-Term Marketable Securities		0
Long-Term Marketable Securities		39
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		10,487
Gross Unrealized Gains		0
Gross Unrealized Losses		(2)
Aggregate Fair Value		10,485
Short-Term Marketable Securities		10,485
Long-Term Marketable Securities		0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,017,036	1,077,387
Gross Unrealized Gains	668	454
Gross Unrealized Losses	(1,179)	(2,132)
Aggregate Fair Value	1,016,525	1,075,709
Short-Term Marketable Securities	392,335	424,777
Long-Term Marketable Securities	624,190	650,932
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	230,598	303,808
Gross Unrealized Gains	257	20
Gross Unrealized Losses	(25)	(427)
Aggregate Fair Value	230,830	303,401
Short-Term Marketable Securities	19,047	84,380
Long-Term Marketable Securities	$ 211,783	$ 219,021

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

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