Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of cash, cash equivalents and marketable securities
Amortized Cost	$ 2,057,652,000	$ 1,413,220,000
Gross Unrealized Gains	31,608,000	21,422,000
Gross Unrealized Losses	(594,000)	(85,000)
Fair Value	2,088,666,000	1,434,557,000
Other than temporary impairment losses	0	0	$ 0
Realized gains or losses	0	0	$ 0
Cash and money market funds
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,595,377,000	1,183,945,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,595,377,000	1,183,945,000
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	12,679,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,000)
Fair Value	12,678,000
Commercial paper
Summary of cash, cash equivalents and marketable securities
Amortized Cost	57,371,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(14,000)
Fair Value	57,357,000
Total cash and cash equivalents
Summary of cash, cash equivalents and marketable securities
Amortized Cost	1,665,427,000	1,183,945,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15,000)	0
Fair Value	1,665,412,000	1,183,945,000
Corporate equity securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	43,213,000	43,213,000
Gross Unrealized Gains	31,608,000	21,347,000
Gross Unrealized Losses	0	0
Fair Value	74,821,000	64,560,000
Government-sponsored enterprise securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	2,304,000	15,506,000
Gross Unrealized Gains	0	2,000
Gross Unrealized Losses	(1,000)	0
Fair Value	2,303,000	15,508,000
Corporate debt securities (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	215,639,000	111,225,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(363,000)	(85,000)
Fair Value	215,276,000	111,140,000
Corporate debt securities (matures after 1 year through 5 years)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	50,697,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(106,000)
Fair Value	50,591,000
Commercial paper (matures within 1 year)
Summary of cash, cash equivalents and marketable securities
Amortized Cost	80,372,000	59,331,000
Gross Unrealized Gains	0	73,000
Gross Unrealized Losses	(109,000)	0
Fair Value	80,263,000	59,404,000
Total marketable securities
Summary of cash, cash equivalents and marketable securities
Amortized Cost	392,225,000	229,275,000
Gross Unrealized Gains	31,608,000	21,422,000
Gross Unrealized Losses	(579,000)	(85,000)
Fair Value	$ 423,254,000	$ 250,612,000