Stock Compensation Plans - Summary of Stock Option Activity (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Options
Outstanding, Beginning balance (in shares) | shares	6,083,482
Granted (in shares) | shares	2,459,728
Exercised (in shares) | shares	(134)
Forfeited or expired (in shares) | shares	(43,926)
Outstanding, Ending balance (in shares) | shares	8,499,150
Options vested and expected to vest, Ending balance (in shares) | shares	8,180,579
Options exercisable, Ending balance (in shares) | shares	4,297,119
Weighted- Average Exercise Price
Outstanding, Beginning balance (usd per share) | $ / shares	$ 6.76
Granted (usd per share) | $ / shares	6.01
Exercised (usd per share) | $ / shares	5.80
Forfeited or expired (usd per share) | $ / shares	6.74
Outstanding, Ending balance (usd per share) | $ / shares	6.54
Options vested and expected to vest, Weighted-Average Exercise Price, Ending balance (usd per share) | $ / shares	6.58
Options exercisable, Weighted-Average Exercise Price, Ending balance (usd per share) | $ / shares	$ 7.38
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding, Weighted-Average Remaining Contractual Term, Ending balance	7 years 4 months 6 days
Options vested and expected to vest , Weighted-Average Remaining Contractual Term, Ending balance	7 years 3 months 11 days
Options exercisable, Weighted-Average Remaining Contractual Term, Ending balance	5 years 9 months 7 days
Outstanding, Aggregate Intrinsic Value, Ending balance | $	$ 10,957,292
Options vested and expected to vest , Aggregate Intrinsic Value, Ending balance | $	10,531,284
Options exercisable, Aggregate Intrinsic Value, Ending balance | $	$ 5,095,729