Stock Compensation Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017
Options
Options, Outstanding, Beginning balance (shares)		4,841,274
Granted (in shares)		1,405,054
Exercised (shares)		(2,889)
Forfeited and expired (shares)		(159,957)
Options, Outstanding, Ending balance (shares)	4,841,274	6,083,482
Options vested and expected to vest, Ending balance (shares)		5,980,525
Options exercisable, Ending balance (shares)		3,562,490
Weighted- Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning balance (usd per share)		$ 7.78
Granted, Weighted-Average Exercise Price (usd per share)		3.28
Exercised, Weighted-Average Exercise Price (usd per share)		1.84
Forfeited, Weighted-Average Exercise Price (usd per share)		7.38
Outstanding, Weighted-Average Exercise Price, Ending balance (usd per share)	$ 7.78	6.76
Options vested and expected to vest, Weighted-Average Exercise Price, Ending balance (usd per share)		6.79
Options exercisable, Weighted-Average Exercise Price, Ending balance (usd per share)		$ 7.94
Weighted- Average Remaining Contractual Term (Years)
Outstanding, Weighted-Average Remaining Contractual Term (Years), Ending balance	7 years
Options vested and expected to vest , Weighted-Average Remaining Contractual Term (Years), Ending balance	6 years 11 months 1 day
Options exercisable, Weighted-Average Remaining Contractual Term (Years), Ending balance	5 years 8 months 12 days
Outstanding, Aggregate Intrinsic Value, Ending balance		$ 6,709
Options vested and expected to vest , Aggregate Intrinsic Value, Ending balance		6,530
Options exercisable, Aggregate Intrinsic Value, Ending balance		$ 2,491