Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (11,475)	$ (10,694)	$ (12,871)	$ (13,039)	$ (4,714)	$ (10,632)	$ (2,594)	$ (2,623)	$ (48,079)	$ (20,563)	$ (8,517)
Accretion of convertible preferred stock										(2,566)	(2,054)
Net loss attributable to common stockholders	$ (11,475)	$ (10,694)	$ (12,871)	$ (13,039)	$ (4,927)	$ (11,652)	$ (3,270)	$ (3,280)	$ (48,079)	$ (23,129)	$ (10,571)
Denominator:
Weighted average shares of common stock outstanding (in shares)									21,415,733	6,637,678	2,730,427
Less: Weighted average shares of unvested restricted common stock outstanding (in shares)										(530,636)	(1,010,345)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders, basic and diluted (in shares)									21,415,733	6,107,042	1,720,082
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.49)	$ (0.50)	$ (0.62)	$ (0.65)	$ (0.28)	$ (5.12)	$ (1.52)	$ (1.61)	$ (2.25)	$ (3.79)	$ (6.15)