Summary of Significant Accounting Policies - Net Loss Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Anti-dilutive securities
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	6,093,309	4,287,055
Common stock warrants
Anti-dilutive securities
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	242,779	242,779
Unvested restricted stock units
Anti-dilutive securities
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	147,554	264,361
Common stock options
Anti-dilutive securities
Anti-dilutive securities excluded from the calculation of diluted net loss per share (in shares)	5,702,976	3,779,915