Summary of Significant Accounting Policies - Reconciliation of Preferred Stock Warrant Liabilities (Details) - Preferred stock warrant - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of preferred stock warrant liabilities:
Balance at beginning of period	$ 157	$ 53	$ 248
Initial fair value of preferred stock warrants issued	103	4	0
Reclassified to equity	(855)	0	0
Change in fair value of preferred stock warrants	595	100	(195)
Balance at end of period	$ 0	$ 157	$ 53