Stock Benefit Plans and Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Outstanding, Beginning balance	1,155
Shares, Granted	2,878
Shares, Forfeited	(877)
Shares Outstanding, Ending balance	3,156	1,155
Shares, Options vested and exercisable	535
Shares, Options vested and expected to vest	2,776
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, Outstanding, Beginning balance	$ 12.17
Weighted average exercise price, Granted	2.00
Weighted average exercise price, Forfeited	7.08
Weighted average exercise price, Outstanding, Ending balance	4.31	$ 12.17
Weighted average exercise price, Options vested and exercisable	14.05
Weighted average exercise price, Options vested and expected to vest	$ 4.59
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average remaining contractual term, Outstanding, balance	8 years 3 months 10 days	7 years 9 months
Weighted average remaining contractual term, Options vested and exercisable	8 years 2 months 1 day
Weighted average remaining contractual term, Options vested and expected to vest	4 years 6 months 18 days
Aggregate intrinsic value, Outstanding, Beginning balance	$ 0
Aggregate intrinsic value, Outstanding, Ending balance	1,841	$ 0
Aggregate intrinsic value, Options vested and exercisable	45
Aggregate intrinsic value, Options vested and expected to vest	$ 1,573