Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial assets
Total financial assets	$ 420,230	$ 155,986
Financial liabilities
Total financial liabilities	2,602	336
Convertible Preferred Stock Warrant Liability
Financial liabilities
Total financial liabilities	2,602	336
Level 1
Financial assets
Total financial assets	35,695	45,998
Level 2
Financial assets
Total financial assets	384,535	109,988
Level 3
Financial liabilities
Total financial liabilities	2,602	336
Level 3 | Convertible Preferred Stock Warrant Liability
Financial liabilities
Total financial liabilities	2,602	336
Money Market Funds
Financial assets
Total financial assets	35,695	45,998
Money Market Funds | Level 1
Financial assets
Total financial assets	35,695	45,998
Commercial Paper
Financial assets
Total financial assets	88,161	16,887
Commercial Paper | Level 2
Financial assets
Total financial assets	88,161	16,887
U.S. Government Debt and Agency Securities
Financial assets
Total financial assets	273,713	85,623
U.S. Government Debt and Agency Securities | Level 2
Financial assets
Total financial assets	273,713	85,623
Corporate Bonds
Financial assets
Total financial assets	22,661	7,478
Corporate Bonds | Level 2
Financial assets
Total financial assets	$ 22,661	$ 7,478