Fair Value Measurements - Marketable Securities (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,267,260,000	$ 1,391,721,000
Gross Unrealized Gains	699,000	474,000
Gross Unrealized Losses	(1,171,000)	(2,561,000)
Aggregate Fair Value	1,266,788,000	1,389,634,000
Short-Term Marketable Securities	386,055,000	519,642,000
Long-Term Marketable Securities	880,733,000	869,992,000
Available-for-sale marketable securities, continuous unrealized loss position for more than 12 months	64,800,000	0
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		39,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Aggregate Fair Value		39,000
Short-Term Marketable Securities		0
Long-Term Marketable Securities		39,000
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,498,000	10,487,000
Gross Unrealized Gains	1,000	0
Gross Unrealized Losses	0	(2,000)
Aggregate Fair Value	3,499,000	10,485,000
Short-Term Marketable Securities	3,499,000	10,485,000
Long-Term Marketable Securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,015,164,000	1,077,387,000
Gross Unrealized Gains	588,000	454,000
Gross Unrealized Losses	(1,124,000)	(2,132,000)
Aggregate Fair Value	1,014,628,000	1,075,709,000
Short-Term Marketable Securities	335,212,000	424,777,000
Long-Term Marketable Securities	679,416,000	650,932,000
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	248,598,000	303,808,000
Gross Unrealized Gains	110,000	20,000
Gross Unrealized Losses	(47,000)	(427,000)
Aggregate Fair Value	248,661,000	303,401,000
Short-Term Marketable Securities	47,344,000	84,380,000
Long-Term Marketable Securities	$ 201,317,000	$ 219,021,000