Summary of Significant Accounting Policies - Reconciliation of Preferred Stock Warrant Liabilities (Details) - Preferred stock warrant - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of preferred stock warrant liabilities:
Balance at beginning of period	$ 978	$ 57	$ 157	$ 53
Initial fair value of preferred stock warrants issued			103	4
Reclassified to equity	(855)		(855)
Change in fair value of preferred stock warrants	$ (123)		$ 595
Balance at end of period		$ 57		$ 57