Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,267,887	$ 1,237,178
Gross Unrealized Gains	2,540	114
Gross Unrealized Losses	(147)	(3,944)
Aggregate Fair Value	1,270,280	1,233,348
Short-Term Marketable Securities	542,062	460,088
Long-Term Marketable Securities	728,218	773,260
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,376	2,491
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(4)
Aggregate Fair Value	18,363	2,487
Short-Term Marketable Securities	18,363	2,487
Long-Term Marketable Securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	979,799	995,100
Gross Unrealized Gains	2,178	73
Gross Unrealized Losses	(111)	(3,365)
Aggregate Fair Value	981,866	991,808
Short-Term Marketable Securities	458,619	432,585
Long-Term Marketable Securities	523,247	559,223
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	51,400	71,400
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	269,712	239,587
Gross Unrealized Gains	362	41
Gross Unrealized Losses	(23)	(575)
Aggregate Fair Value	270,051	239,053
Short-Term Marketable Securities	65,080	25,016
Long-Term Marketable Securities	$ 204,971	$ 214,037

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

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