Summary of Significant Accounting Policies - Dilutive Common Shares (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net loss per share
Potentially dilutive common shares that have not been included in the net loss per common share calculations because the effect would have been anti-dilutive	10,952,880	10,268,710	8,129,739
Stock options
Net loss per share
Potentially dilutive common shares that have not been included in the net loss per common share calculations because the effect would have been anti-dilutive	10,856,212	10,218,710	8,110,239
Restricted stock
Net loss per share
Potentially dilutive common shares that have not been included in the net loss per common share calculations because the effect would have been anti-dilutive	96,668	50,000	19,500