Net Loss per Share Attributable to Common Stock - Computation of Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Numerator:
Net loss	$ (36,439)	$ (28,066)	$ (15,002)
Accretion of redeemable convertible preferred stock	0	(24,770)
Net loss attributable to common stock—basic	(36,439)	(52,836)
Net loss attributable to common stock—diluted	$ (36,439)	$ (52,836)
Denominator:
Weighted average shares used to compute net income (loss) per share attributable to common stock—basic and diluted (in shares)	82,010,913	14,767,478
Net loss per share attributable to common stock—basic and diluted (in dollars per share)	$ (0.44)	$ (3.58)