Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,042,336	$ 1,387,229
Gross Unrealized Gains	246	88
Gross Unrealized Losses	(120)	(211)
Fair Value	1,042,462	1,387,106
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	714,768	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	714,768	625,259
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	582,352	625,259
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	582,352	625,259
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	85,994
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	85,994
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	34,889
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	34,889
Corporate debt securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	11,533
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	11,533
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	327,568	761,970
Gross Unrealized Gains	246	88
Gross Unrealized Losses	(120)	(211)
Fair Value	327,694	761,847
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	87,176	463,788
Gross Unrealized Gains	0	14
Gross Unrealized Losses	(14)	(52)
Fair Value	87,162	463,750
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	98,877	51,674
Gross Unrealized Gains	246	72
Gross Unrealized Losses	0	0
Fair Value	99,123	51,746
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	141,515	196,065
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(106)	(66)
Fair Value	$ 141,409	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