Warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Stock-based award plans (Details) - shares	Jan. 01, 2019	Sep. 30, 2019	Dec. 31, 2018
2007 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Common stock available for future awards (in shares)		0
2017 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Shares authorized under the plan (in shares)		1,042,314
Shares available for grant under the plan (in shares)		207,589
Annual increase in the shares available for grant under the plan (as a percent of shares of common stock outstanding)		4.00%
Increase in the shares available for grant under the plan (in shares)	894,761
2017 ESPP
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Shares authorized under the plan (in shares)		649,223
Shares available for grant under the plan (in shares)		612,470
Annual increase in the shares available for grant under the plan (as a percent of shares of common stock outstanding)		1.00%
Increase in the shares available for grant under the plan (in shares)	223,690
Outstanding stock options
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)		2,662,334	2,476,911
Outstanding stock options | 2007 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)		1,324,255
Outstanding stock options | 2017 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)		1,338,079
Shares authorized under the plan (in shares)		2,490,290
Unvested restricted stock units | 2017 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)		422,336