Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,071,110	$ 1,042,336
Gross Unrealized Gains	351	246
Gross Unrealized Losses	(25)	(120)
Fair Value	1,071,436	1,042,462
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	605,866	714,768
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	605,866	714,768
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	605,866	582,352
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	605,866	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		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	465,244	327,568
Gross Unrealized Gains	351	246
Gross Unrealized Losses	(25)	(120)
Fair Value	465,570	327,694
U.S. Treasury securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	33,138
Gross Unrealized Gains	4
Gross Unrealized Losses	0
Fair Value	33,142
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	131,868	87,176
Gross Unrealized Gains	35	0
Gross Unrealized Losses	(3)	(14)
Fair Value	131,900	87,162
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	164,575	98,877
Gross Unrealized Gains	278	246
Gross Unrealized Losses	0	0
Fair Value	164,853	99,123
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	135,663	141,515
Gross Unrealized Gains	34	0
Gross Unrealized Losses	(22)	(106)
Fair Value	$ 135,675	$ 141,409