Basis of Presentation and Summary of Significant Accounting Policies - Antidilutive shares excluded from earnings per share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securites excluded from computation of earnings per share	8,023,711	31,542,193
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securites excluded from computation of earnings per share	6,138,373	4,583,148
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securites excluded from computation of earnings per share	1,866,831	1,890,489
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securites excluded from computation of earnings per share	18,507	0
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securites excluded from computation of earnings per share	0	25,068,556