Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,206,742	$ 1,237,178
Gross Unrealized Gains	1,492	114
Gross Unrealized Losses	(533)	(3,944)
Aggregate Fair Value	1,207,701	1,233,348
Short-Term Marketable Securities	524,958	460,088
Long-Term Marketable Securities	682,743	773,260
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,495	2,491
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(4)
Aggregate Fair Value	2,494	2,487
Short-Term Marketable Securities	2,494	2,487
Long-Term Marketable Securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	962,932	995,100
Gross Unrealized Gains	1,203	73
Gross Unrealized Losses	(506)	(3,365)
Aggregate Fair Value	963,629	991,808
Short-Term Marketable Securities	479,400	432,585
Long-Term Marketable Securities	484,229	559,223
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	76,400	71,400
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	241,315	239,587
Gross Unrealized Gains	289	41
Gross Unrealized Losses	(26)	(575)
Aggregate Fair Value	241,578	239,053
Short-Term Marketable Securities	43,064	25,016
Long-Term Marketable Securities	$ 198,514	$ 214,037

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

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