Net Loss Per Share - Weighted Average Shares (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	$ (47,015)	$ (39,390)	$ (32,581)	$ (26,362)	$ (21,239)	$ (14,411)	$ (118,986)	$ (62,012)
Denominator:
Weighted average common shares outstanding, basic and diluted	79,115,305			62,311,111			78,357,791	26,662,233
Net loss per share, basic and diluted	$ (0.59)			$ (0.42)			$ (1.52)	$ (2.33)