Investments - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Total available-for-sale securities
Amortized Cost	$ 48,205	$ 47,807
Gross Unrealized Losses	(16)	(27)
Estimated Fair Value	48,189	47,780
Available-for-sale securities in a loss position
Estimated Fair Value	45,700	44,781
Gross Unrealized Losses	(16)	(27)
Short-term investments in a continuous loss position
Investments that had been in a continuous loss position for more than 12 months	0	0
U.S. treasuries
Total available-for-sale securities
Amortized Cost	37,191	33,914
Gross Unrealized Losses	(7)	(19)
Estimated Fair Value	37,184	33,895
Available-for-sale securities in a loss position
Estimated Fair Value	37,184	33,895
Gross Unrealized Losses	(7)	(19)
Corporate debt
Total available-for-sale securities
Amortized Cost	8,525	10,894
Gross Unrealized Losses	(9)	(8)
Estimated Fair Value	8,516	10,886
Available-for-sale securities in a loss position
Estimated Fair Value	8,516	10,886
Gross Unrealized Losses	(9)	(8)
Commercial paper
Total available-for-sale securities
Amortized Cost	2,489	2,999
Estimated Fair Value	$ 2,489	$ 2,999