Available-for-Sale Securities (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) item
Available-for-Sale Securities
Amortized Cost	$ 136,875		$ 136,875		$ 178,193
Gross Unrealized Gains	62		62		2
Gross Unrealized Losses	(1)		(1)		(88)
Fair Value	$ 136,936		$ 136,936		$ 178,107
Contractual maturity period, maximum			1 year
Number of investments classified as available-for-sale securities in an unrealized loss position (in investments) | item	4		4		32
Number of investments classified as available-for-sale securities in an unrealized loss position for more than twelve months (in investments) | item	0		0		0
Aggregate fair value of securities none of which had been in an unrealized loss position for more than twelve months	$ 5,200		$ 5,200		$ 167,600
Proceeds from sales of available-for-sale securities	0	$ 0	0	$ 0
U.S. Treasury securities
Available-for-Sale Securities
Amortized Cost	40,376		40,376		50,124
Gross Unrealized Gains	28		28
Gross Unrealized Losses					(33)
Fair Value	40,404		40,404		50,091
U.S. government-sponsored securities
Available-for-Sale Securities
Amortized Cost	96,499		96,499		128,069
Gross Unrealized Gains	34		34		2
Gross Unrealized Losses	(1)		(1)		(55)
Fair Value	$ 96,532		$ 96,532		$ 128,016