Stock-based Compensation - Reserved Shares (Details) - shares	Mar. 31, 2020	Dec. 31, 2019
Stock-based compensation
Common stock shares reserved for future issuance	9,961,500	8,488,405
2017 Employee Stock Purchase Plan
Stock-based compensation
Common stock shares reserved for future issuance	5,904,299	4,429,849
ESPP
Stock-based compensation
Common stock shares reserved for future issuance	523,860	438,307
Employee and Non-Employee Stock Options
Stock-based compensation
Common stock shares reserved for future issuance	709,500	705,500
Warrants
Stock-based compensation
Common stock shares reserved for future issuance	384,163	384,163
2009 plan
Stock-based compensation
Common stock shares reserved for future issuance	2,439,678	2,530,586