Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Liabilities
Redeemable convertible preferred stock warrants	$ 0	$ 422
Money market funds
Assets
Investments	15,223	22,115
U.S. treasury securities
Assets
Investments	66,489
Corporate bonds
Assets
Investments	19,720
Commercial paper
Assets
Investments	18,529
Level 1
Liabilities
Redeemable convertible preferred stock warrants	0	0
Level 1 | Money market funds
Assets
Investments	15,223	22,115
Level 1 | U.S. treasury securities
Assets
Investments	66,489
Level 1 | Corporate bonds
Assets
Investments	0
Level 1 | Commercial paper
Assets
Investments	0
Level 2
Liabilities
Redeemable convertible preferred stock warrants	0	0
Level 2 | Money market funds
Assets
Investments	0	0
Level 2 | U.S. treasury securities
Assets
Investments	0
Level 2 | Corporate bonds
Assets
Investments	19,720
Level 2 | Commercial paper
Assets
Investments	18,529
Level 3
Liabilities
Redeemable convertible preferred stock warrants	0	422
Level 3 | Money market funds
Assets
Investments	0	$ 0
Level 3 | U.S. treasury securities
Assets
Investments	0
Level 3 | Corporate bonds
Assets
Investments	0
Level 3 | Commercial paper
Assets
Investments	$ 0