Stock-based Compensation - Reserved Shares (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Stock-based compensation
Common stock shares reserved for future issuance	8,643,425	6,799,486
2017 Employee Stock Purchase Plan
Stock-based compensation
Common stock shares reserved for future issuance	4,485,323	3,130,910
Stock Options
Stock-based compensation
Common stock shares reserved for future issuance	673,500	498,000
ESPP
Stock-based compensation
Common stock shares reserved for future issuance	561,971	223,341
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	2,538,468	2,563,072