Common Stock, warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Stock-based award plans (Details) - shares		3 Months Ended
	Jan. 01, 2020	Sep. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Common stock available for future awards (in shares)				3,809,706	5,348,442
2007 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Shares authorized under the plan (in shares)		4,350,035					3,852,213	3,229,935
Additional shares authorized		497,822	622,227
Shares available for grant under the plan (in shares)				0
Annual increase in the shares available for grant under the plan (as a percent of shares of common stock outstanding)				4.00%
2017 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Shares authorized under the plan (in shares)						1,042,314
Additional shares authorized	894,761
Shares available for grant under the plan (in shares)				270,143	4,393
2017 ESPP
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Shares authorized under the plan (in shares)					425,533
Shares available for grant under the plan (in shares)				612,572
Annual increase in the shares available for grant under the plan (as a percent of shares of common stock outstanding)					1.00%
Outstanding stock options
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)				2,507,062	2,476,911
Outstanding stock options | 2007 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Options outstanding (in shares)				1,221,746	2,249,843
Common stock issued and outstanding pursuant to the exercise of options (in shares)					571,838
Common stock available for future awards (in shares)					399,379
Shares authorized under the plan (in shares)						2,490,290
Restricted and unrestricted stock awards | 2007 Plan
Warrants, stock-based compensation, stock options, restricted stock and restricted stock units
Common stock issued and outstanding pursuant to restricted or unrestricted stock awards (in shares)					1,128,975