Net Loss per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (43,875)	$ (28,880)	$ (85,444)	$ (52,764)	$ (137,481)	$ (71,894)
Net loss:	$ (43,875)	$ (28,880)	$ (85,444)	$ (52,764)	$ (137,481)	$ (71,894)
Denominator:
Weighted average common shares outstanding, basic and diluted	37,697	31,826	37,659	31,765
Net loss per share, basic and diluted	$ (1.16)	$ (0.91)	$ (2.27)	$ (1.66)