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	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 23,103	$ 12,472
Liabilities
Derivative liability		3,028
Preferred stock warrant liability	5,990	1,656
Total liabilities		4,684
Level 1
Assets
Cash equivalents	23,103	12,472
Level 3
Liabilities
Derivative liability		3,028
Preferred stock warrant liability	$ 5,990	1,656
Total liabilities		$ 4,684