Short-term Investments - Available-For-Sale Securities Reported At Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Gross Unrealized Gains	$ 1,034	$ 557
Level 2 securities, Gross Unrealized Losses	(476)	(295)
Total, Amortized Cost	2,617,590	1,613,928
Total, Estimated Fair Value	2,618,148	1,614,190
Level 1 Securities [Member] | Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost	115,316	111,759
Cash and money market funds, Estimated Fair Value	115,316	111,759
Level 2 Securities [Member] | Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	251,192	295,500
Level 2 securities, Gross Unrealized Gains	406	432
Level 2 securities, Gross Unrealized Losses	(467)	(199)
Level 2 securities, Estimated Fair Value	251,131	295,733
Level 2 Securities [Member] | Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	212,256	168,749
Level 2 securities, Gross Unrealized Gains	599	120
Level 2 securities, Gross Unrealized Losses	(9)	(95)
Level 2 securities, Estimated Fair Value	212,846	168,774
Level 2 Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost		112
Level 2 securities, Gross Unrealized Gains		0
Level 2 securities, Gross Unrealized Losses		0
Level 2 securities, Estimated Fair Value		112
Level 2 Securities [Member] | Certificates of deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	3,600	3,600
Level 2 securities, Gross Unrealized Gains	0	0
Level 2 securities, Gross Unrealized Losses	0	0
Level 2 securities, Estimated Fair Value	3,600	3,600
Level 2 Securities [Member] | Agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Level 2 securities, Amortized Cost	26,599	26,665
Level 2 securities, Gross Unrealized Gains	29	5
Level 2 securities, Gross Unrealized Losses	0	(1)
Level 2 securities, Estimated Fair Value	26,628	26,669
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and money market funds, Amortized Cost	2,008,627	1,007,543
Cash and money market funds, Estimated Fair Value	$ 2,008,627	$ 1,007,543