Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Summary of Investments
Available-for-sale securities, Amortized Cost	$ 238,568	$ 24,914
Available-for-sale securities, Gross Unrealized Gains	5	13
Available-for-sale securities, Gross Unrealized Losses	(947)	(37)
Available-for-sale securities, Estimated Fair Value	237,626	24,890
Corporate debt securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	27,964	11,656
Available-for-sale securities due in one to three years, Amortized Cost	173,652	13,258
Available-for-sale securities due in one year or less, Gross Unrealized Gains		3
Available-for-sale securities due in one to three years, Gross Unrealized Gains	3	10
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(38)	(4)
Available-for-sale securities due in one to three years, Gross Unrealized Losses	(904)	(33)
Available-for-sale securities due in one year or less, Estimated Fair Value	27,926	11,655
Available-for-sale securities due in one to three years, Estimated Fair Value	172,751	$ 13,235
Commercial Paper
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	34,452
Available-for-sale securities due in one year or less, Gross Unrealized Gains	2
Available-for-sale securities due in one year or less, Gross Unrealized Losses	(5)
Available-for-sale securities due in one year or less, Estimated Fair Value	34,449
Municipal securities
Summary of Investments
Available-for-sale securities due in one year or less, Amortized Cost	2,500
Available-for-sale securities due in one year or less, Estimated Fair Value	$ 2,500