Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,413,220,000	$ 1,042,336,000
Gross Unrealized Gains	21,422,000	246,000
Gross Unrealized Losses	(85,000)	(120,000)
Fair Value	1,434,557,000	1,042,462,000
Other than temporary impairment losses	0	0	$ 0
Realized gains or losses	0	0	$ 0
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,183,945,000	714,768,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,183,945,000	714,768,000
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,183,945,000	582,352,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,183,945,000	582,352,000
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		85,994,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		85,994,000
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost		34,889,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		34,889,000
Corporate debt securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost		11,533,000
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		11,533,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	229,275,000	327,568,000
Gross Unrealized Gains	21,422,000	246,000
Gross Unrealized Losses	(85,000)	(120,000)
Fair Value	250,612,000	327,694,000
Government-sponsored enterprise securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	15,506,000	87,176,000
Gross Unrealized Gains	2,000	0
Gross Unrealized Losses	0	(14,000)
Fair Value	15,508,000	87,162,000
Corporate equity securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	43,213,000
Gross Unrealized Gains	21,347,000
Gross Unrealized Losses	0
Fair Value	64,560,000
Commercial paper (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	59,331,000	98,877,000
Gross Unrealized Gains	73,000	246,000
Gross Unrealized Losses	0	0
Fair Value	59,404,000	99,123,000
Corporate debt securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	111,225,000	141,515,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(85,000)	(106,000)
Fair Value	$ 111,140,000	$ 141,409,000