Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,005,767	$ 1,387,229
Gross Unrealized Gains	142	88
Gross Unrealized Losses	(79)	(211)
Fair Value	1,005,830	1,387,106
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	756,250	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	756,250	625,259
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	707,141	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	707,141	625,259
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	25,109	51,674
Gross Unrealized Gains	0	72
Gross Unrealized Losses	0	0
Fair Value	25,109	51,746
Corporate Debt Securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	24,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	24,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	249,517	761,970
Gross Unrealized Gains	142	88
Gross Unrealized Losses	(79)	(211)
Fair Value	249,580	761,847
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost		463,788
Gross Unrealized Gains		14
Gross Unrealized Losses		(52)
Fair Value		463,750
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	140,131
Gross Unrealized Gains	114
Gross Unrealized Losses	0
Fair Value	140,245
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	109,386	196,065
Gross Unrealized Gains	28	2
Gross Unrealized Losses	(79)	(66)
Fair Value	$ 109,335	196,001
Corporate debt securities (due after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost		50,443
Gross Unrealized Gains		0
Gross Unrealized Losses		(93)
Fair Value		$ 50,350