Summary of Significant Accounting Policies - Net Loss Per Share (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Numerator: Net loss attributable to common shareholders	$ (10,005,597)	$ (10,268,578)
Adjustment for (gain) loss from change in fair value of derivative financial instruments—warrants	0	(533,750)
Net loss used for diluted loss per share	$ (10,005,597)	$ (10,802,328)
Denominator for basic and diluted net loss per share:
Weighted-average shares used to compute basic loss per share	30,961,014	29,755,184
Adjustments to reflect assumed exercise of warrants	0	353,193
Weighted-average shares used to compute diluted net loss per share	30,961,014	30,108,377
Net loss per share attributable to common stockholders:
Net loss per common share - basic (in USD per share)	$ (0.32)	$ (0.35)
Net loss per common share - diluted (in USD per share)	$ (0.32)	$ (0.36)