Marketable Securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 1,661,060,000		$ 1,661,060,000		$ 1,413,220,000
Gross Unrealized Gains	7,843,000		7,843,000		21,422,000
Gross Unrealized Losses	(253,000)		(253,000)		(85,000)
Fair Value	1,668,650,000		1,668,650,000		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	1,223,130,000		1,223,130,000		1,183,945,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Fair Value	1,223,130,000		1,223,130,000		1,183,945,000
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,217,134,000		1,217,134,000		1,183,945,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Fair Value	1,217,134,000		1,217,134,000		1,183,945,000
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	5,996,000		5,996,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	5,996,000		5,996,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	437,930,000		437,930,000		229,275,000
Gross Unrealized Gains	7,843,000		7,843,000		21,422,000
Gross Unrealized Losses	(253,000)		(253,000)		(85,000)
Fair Value	445,520,000		445,520,000		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	7,836,000		7,836,000		21,347,000
Gross Unrealized Losses	0		0		0
Fair Value	51,049,000		51,049,000		64,560,000
Commercial paper (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	103,386,000		103,386,000		59,331,000
Gross Unrealized Gains	1,000		1,000		73,000
Gross Unrealized Losses	(40,000)		(40,000)		0
Fair Value	103,347,000		103,347,000		59,404,000
Corporate debt securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	218,216,000		218,216,000		111,225,000
Gross Unrealized Gains	4,000		4,000		0
Gross Unrealized Losses	(143,000)		(143,000)		(85,000)
Fair Value	218,077,000		218,077,000		$ 111,140,000
Corporate debt securities (matures after 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	73,115,000		73,115,000
Gross Unrealized Gains	2,000		2,000
Gross Unrealized Losses	(70,000)		(70,000)
Fair Value	$ 73,047,000		$ 73,047,000