Significant Accounting Policies - Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	7,542,648	7,227,108
Outstanding common stock options
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	6,860,968	6,479,347
Outstanding warrants
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	34,647	34,647
PRSUs
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share		213,758
RSUs
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	616,912	469,705
Employee Stock Purchase Plan
Common stock equivalents
Antidilutive securities excluded from computation of earnings per share	30,121	29,651