Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (55,319)	$ (49,876)	$ (37,565)	$ (32,740)	$ (31,218)	$ (30,229)	$ (142,760)	$ (94,187)
Denominator:
Weighted average shares of common stock outstanding (in shares)	41,364,387			30,982,192			41,296,377	30,938,026
Net loss per share, basic and diluted	$ (1.34)			$ (1.06)			$ (3.46)	$ (3.04)