Assets - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Total available-for-sale securities
Amortized Cost	$ 52,086		$ 52,086		$ 47,807
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(24)		(24)		(27)
Estimated Fair Value	52,063		52,063		47,780
Available-for-sale securities in a loss position
Estimated Fair Value	38,182		38,182		44,781
Gross Unrealized Losses	(24)		(24)		(27)
Investments that had been in a continuous loss position for more than 12 months	0		0		0
Realized gains	0	$ 0	0	$ 0
Realized losses	0		0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	36,538		36,538		33,914
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(12)		(12)		(19)
Estimated Fair Value	36,527		36,527		33,895
Available-for-sale securities in a loss position
Estimated Fair Value	29,621		29,621		33,895
Gross Unrealized Losses	(12)		(12)		(19)
Corporate debt
Total available-for-sale securities
Amortized Cost	8,573		8,573		10,894
Gross Unrealized Losses	(12)		(12)		(8)
Estimated Fair Value	8,561		8,561		10,886
Available-for-sale securities in a loss position
Estimated Fair Value	8,561		8,561		10,886
Gross Unrealized Losses	(12)		(12)		(8)
Commercial paper
Total available-for-sale securities
Amortized Cost	6,975		6,975		2,999
Estimated Fair Value	$ 6,975		$ 6,975		$ 2,999