Fair Value Measurements - Summary of Changes in Fair Value of Convertible Preferred Stock Warrant Liability (Details) - Fair Value Measurements Recurring - Level 3 - Convertible Preferred Stock Warrant Liability - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance, fair value	$ 1,875	$ 1,351	$ 1,204
Issuance of convertible preferred stock warrants	111		147
Change in fair value recorded in other expense, net	3,725	524
Reclassification of unexercised warrant into additional paid-in capital upon the IPO	(5,711)
Ending balance, fair value	$ 0	$ 1,875	$ 1,351