Marketable Securities (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,382,879,000		$ 1,413,220,000
Gross Unrealized Gains	26,163,000		21,422,000
Gross Unrealized Losses	(260,000)		(85,000)
Fair Value	1,408,782,000		1,434,557,000
Other than temporary impairment losses, investments	0	$ 0
Gross realized gains (losses)	0	$ 0
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,003,679,000		1,183,945,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	1,003,679,000		1,183,945,000
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	985,818,000		1,183,945,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	985,818,000		1,183,945,000
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	6,834,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	6,834,000
Corporate debt securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	11,027,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	11,027,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	379,200,000		229,275,000
Gross Unrealized Gains	26,163,000		21,422,000
Gross Unrealized Losses	(260,000)		(85,000)
Fair Value	405,103,000		250,612,000
Government-sponsored enterprise securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost			15,506,000
Gross Unrealized Gains			2,000
Gross Unrealized Losses			0
Fair Value			15,508,000
Corporate equity securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	43,213,000		43,213,000
Gross Unrealized Gains	26,159,000		21,347,000
Gross Unrealized Losses	0		0
Fair Value	69,372,000		64,560,000
Commercial paper (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	132,640,000		59,331,000
Gross Unrealized Gains	1,000		73,000
Gross Unrealized Losses	(91,000)		0
Fair Value	132,550,000		59,404,000
Corporate debt securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	180,465,000		111,225,000
Gross Unrealized Gains	3,000		0
Gross Unrealized Losses	(143,000)		(85,000)
Fair Value	180,325,000		$ 111,140,000
Corporate Debt Securities, (matures after 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	22,882,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(26,000)
Fair Value	$ 22,856,000