Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Investments	$ 308,792	$ 237,626
Summary of Investments
Available-for-sale securities, Amortized Cost	308,468	238,568
Available-for-sale securities, Gross Unrealized Gains	431	5
Available-for-sale securities, Gross Unrealized Losses	(107)	(947)
Available-for-sale securities, Estimated Fair Value	308,792	237,626
Corporate debt securities
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	116,674	27,964
Available-for-sale securities, due in one to three years, Amortized Cost	135,719	173,652
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	7	0
Available-for-sale securities, due in one to three years, Gross Unrealized Gains	398	3
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	(84)	(38)
Available-for-sale securities, due in one to three years, Gross Unrealized Losses	(20)	(904)
Available-for-sale securities, due in one year or less, Estimated Fair Value	116,597	27,926
Available-for-sale securities, due in one to three years, Estimated Fair Value	136,097	172,751
U.S. treasury and government agency securities
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	1,032
Available-for-sale securities, due in one to three years, Amortized Cost	8,891
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	1
Available-for-sale securities, due in one to three years, Gross Unrealized Gains	25
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	0
Available-for-sale securities, due in one to three years, Gross Unrealized Losses	(2)
Available-for-sale securities, due in one year or less, Estimated Fair Value	1,033
Available-for-sale securities, due in one to three years, Estimated Fair Value	8,914
Commercial paper
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	31,852	34,452
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0	2
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	0	(5)
Available-for-sale securities, due in one year or less, Estimated Fair Value	31,852	34,449
Certificates of deposit
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	11,800
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	0
Available-for-sale securities, due in one year or less, Estimated Fair Value	11,800
Municipal securities
Summary of Investments
Available-for-sale securities, due in one year or less, Amortized Cost	2,500	2,500
Available-for-sale securities, due in one year or less, Gross Unrealized Gains	0	0
Available-for-sale securities, due in one year or less, Gross Unrealized Losses	(1)	0
Available-for-sale securities, due in one year or less, Estimated Fair Value	$ 2,499	$ 2,500