Investments - Schedule of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Total	$ 75,600	$ 100,389
Gross Unrealized Gains	22	2
Gross Unrealized Losses	(72)	(173)
Fair Value	75,550	100,218
Certificates of deposit
Investment [Line Items]
Total	8,817	10,787
Gross Unrealized Gains	9	0
Gross Unrealized Losses	(17)	0
Fair Value	8,809	10,787
U.S. Treasury securities
Investment [Line Items]
Total	25,401	22,185
Gross Unrealized Gains	8	1
Gross Unrealized Losses	(22)	(66)
Fair Value	25,387	22,120
U.S. Agency securities
Investment [Line Items]
Total	6,521	8,024
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(20)	(45)
Fair Value	6,501	7,980
Commercial paper
Investment [Line Items]
Total	8,463	17,025
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	8,463	17,025
Asset-backed securities
Investment [Line Items]
Total	4,390	12,007
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	(9)
Fair Value	4,391	11,998
Corporate notes and bonds
Investment [Line Items]
Total	22,008	30,361
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(13)	(53)
Fair Value	$ 21,999	$ 30,308