Marketable Securities (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2015
Summary of cash, cash equivalents and marketable securities
Other than Temporary Impairment Losses, Investments	$ 0	$ 0
Amortized Cost	1,128,211,000	1,128,211,000	$ 1,042,336,000
Gross Unrealized Gains	312,000	312,000	246,000
Gross Unrealized Losses	(82,000)	(82,000)	(120,000)
Fair Value	1,128,441,000	1,128,441,000	1,042,462,000
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	0	0
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	719,692,000	719,692,000	714,768,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	719,692,000	719,692,000	714,768,000
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	719,692,000	719,692,000	582,352,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	719,692,000	719,692,000	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	408,519,000	408,519,000	327,568,000
Gross Unrealized Gains	312,000	312,000	246,000
Gross Unrealized Losses	(82,000)	(82,000)	(120,000)
Fair Value	408,749,000	408,749,000	327,694,000
Government-sponsored enterprise securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	130,404,000	130,404,000	87,176,000
Gross Unrealized Gains	29,000	29,000	0
Gross Unrealized Losses	(2,000)	(2,000)	(14,000)
Fair Value	130,431,000	130,431,000	87,162,000
Commercial paper (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	131,099,000	131,099,000	98,877,000
Gross Unrealized Gains	277,000	277,000	246,000
Gross Unrealized Losses	0	0	0
Fair Value	131,376,000	131,376,000	99,123,000
Corporate debt securities (due within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	147,016,000	147,016,000	141,515,000
Gross Unrealized Gains	6,000	6,000	0
Gross Unrealized Losses	(80,000)	(80,000)	(106,000)
Fair Value	$ 146,942,000	$ 146,942,000	$ 141,409,000