Net Loss Per Share - Computation of Net (Loss) Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (28,556)	$ (18,293)	$ (20,852)	$ (23,547)	$ (18,104)	$ (17,640)	$ (24,496)	$ (9,530)	$ (91,248)	$ (69,770)	$ (52,642)
Weighted average shares-basic (shares)									85,115,592	72,824,070	64,882,417
Net (loss) income per share-basic ($ per share)	$ (0.29)	$ (0.22)	$ (0.27)	$ (0.30)	$ (0.23)	$ (0.22)	$ (0.36)	$ (0.15)	$ (1.07)	$ (0.96)	$ (0.81)
Weighted average shares-diluted (shares)									85,115,592	72,824,070	64,882,417
Net (loss) income per share-diluted ($ per share)									$ (1.07)	$ (0.96)	$ (0.81)