Net Loss per Share Attributable to Common Stock - Computation of Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator
Net loss	$ (67,180)	$ (36,439)
Weighted average shares:
Weighted average shares - basic and diluted (in shares)	90,610,416	82,010,913
Net loss per share
Net loss per share - basic and diluted (in dollars per share)	$ (0.74)	$ (0.44)