Marketable Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,798,697,000		$ 1,798,697,000		$ 1,413,220,000
Gross Unrealized Gains	13,740,000		13,740,000		21,422,000
Gross Unrealized Losses	(189,000)		(189,000)		(85,000)
Fair Value	1,812,248,000		1,812,248,000		1,434,557,000
Other than temporary impairment losses, investments	0	$ 0	0	$ 0
Gross realized gains (losses)	0	$ 0	0	$ 0
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,384,968,000		1,384,968,000		1,183,945,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(2,000)		(2,000)		0
Fair Value	1,384,966,000		1,384,966,000		1,183,945,000
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,350,891,000		1,350,891,000		1,183,945,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Fair Value	1,350,891,000		1,350,891,000		1,183,945,000
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	14,979,000		14,979,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	14,979,000		14,979,000
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	14,610,000		14,610,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(2,000)		(2,000)
Fair Value	14,608,000		14,608,000
Corporate debt securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	4,488,000		4,488,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	4,488,000		4,488,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	413,729,000		413,729,000		229,275,000
Gross Unrealized Gains	13,740,000		13,740,000		21,422,000
Gross Unrealized Losses	(187,000)		(187,000)		(85,000)
Fair Value	427,282,000		427,282,000		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		43,213,000
Gross Unrealized Gains	13,731,000		13,731,000		21,347,000
Gross Unrealized Losses	0		0		0
Fair Value	56,944,000		56,944,000		64,560,000
Commercial paper (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	75,186,000		75,186,000		59,331,000
Gross Unrealized Gains	1,000		1,000		73,000
Gross Unrealized Losses	(20,000)		(20,000)		0
Fair Value	75,167,000		75,167,000		59,404,000
Corporate debt securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	235,679,000		235,679,000		111,225,000
Gross Unrealized Gains	8,000		8,000		0
Gross Unrealized Losses	(106,000)		(106,000)		(85,000)
Fair Value	235,581,000		235,581,000		$ 111,140,000
Corporate debt securities (matures after 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	59,651,000		59,651,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(61,000)		(61,000)
Fair Value	$ 59,590,000		$ 59,590,000