Fair Value Measurement - Summary of Assets and Liabilities that Require Fair Value Measurements on a Recurring Basis (Details) - Fair Value Measurements on Recurring Basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 59,357	$ 12,472
Liabilities
Derivative liability		3,028
Preferred stock warrant liability		1,656
Total liabilities		4,684
Level 1
Assets
Cash equivalents	$ 59,357	12,472
Level 3
Liabilities
Derivative liability		3,028
Preferred stock warrant liability		1,656
Total liabilities		$ 4,684