Fair Value Measurement and Financial Instruments - Schedule of Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount | Money market funds
Assets
Total assets	$ 11,229	$ 11,149
Fair Value | Money market funds
Assets
Total assets	11,229	11,149
Convertible preferred sock warrant liability | Carrying Amount
Liabilities
Total liabilities	1,893	478
Convertible preferred sock warrant liability | Fair Value
Liabilities
Total liabilities	1,893	478
Quoted Prices In Active Markets (Level 1) | Money market funds
Assets
Total assets	11,229	11,149
Significant Unobservable Inputs (Level 3) | Convertible preferred sock warrant liability
Liabilities
Total liabilities	$ 1,893	$ 478