Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,025,263	$ 1,042,336
Gross Unrealized Gains	359	246
Gross Unrealized Losses	(4)	(120)
Fair Value	1,025,618	1,042,462
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	604,245	714,768
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	604,245	714,768
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	590,688	582,352
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	590,688	582,352
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	13,557	11,533
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	13,557	11,533
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	421,018	327,568
Gross Unrealized Gains	359	246
Gross Unrealized Losses	(4)	(120)
Fair Value	421,373	327,694
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	33,108
Gross Unrealized Gains	0
Gross Unrealized Losses	(4)
Fair Value	33,104
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	126,461	87,176
Gross Unrealized Gains	17	0
Gross Unrealized Losses	0	(14)
Fair Value	126,478	87,162
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	141,243	98,877
Gross Unrealized Gains	319	246
Gross Unrealized Losses	0	0
Fair Value	141,562	99,123
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	120,206	141,515
Gross Unrealized Gains	23	0
Gross Unrealized Losses	0	(106)
Fair Value	$ 120,229	$ 141,409