Significant Accounting Policies - Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	7,553,214	7,518,390
Outstanding common stock options
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	5,309,344	6,265,333
Outstanding warrants
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	722,000	756,647
RSUs
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	1,510,460	478,087
Employee Stock Purchase Plan
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	11,410	18,323