Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets
Total assets measured at fair value	$ 160,352	$ 18,142
Liabilities
Total liabilities measured at fair value		683
Convertible Preferred Stock Warrants Liability
Liabilities
Total liabilities measured at fair value		683
Money Market Funds
Assets
Total assets measured at fair value	160,352	18,142
Level 1
Assets
Total assets measured at fair value	160,352	18,142
Liabilities
Total liabilities measured at fair value		0
Level 1 | Convertible Preferred Stock Warrants Liability
Liabilities
Total liabilities measured at fair value		0
Level 1 | Money Market Funds
Assets
Total assets measured at fair value	160,352	18,142
Level 2
Assets
Total assets measured at fair value	0	0
Liabilities
Total liabilities measured at fair value		0
Level 2 | Convertible Preferred Stock Warrants Liability
Liabilities
Total liabilities measured at fair value		0
Level 2 | Money Market Funds
Assets
Total assets measured at fair value	0	0
Level 3
Assets
Total assets measured at fair value	0	0
Liabilities
Total liabilities measured at fair value		683
Level 3 | Convertible Preferred Stock Warrants Liability
Liabilities
Total liabilities measured at fair value		683
Level 3 | Money Market Funds
Assets
Total assets measured at fair value	$ 0	$ 0