Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss attributable to common stockholders									$ (89,380)	$ (91,579)	$ (54,780)
Denominator:
Weighted average common shares outstanding, basic and diluted									40,449,410	39,846,928	23,532,400
Net loss per share attributable to common stockholders, basic and diluted	$ (0.72)	$ (0.17)	$ (0.69)	$ (0.63)	$ (0.63)	$ (0.46)	$ (0.70)	$ (0.50)	$ (2.21)	$ (2.30)	$ (2.33)