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Equity-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized equity-based compensation expense related to restricted stock	$ 0.8		$ 0.8
Weighted average grant date fair value per share	$ 9.22	$ 16.64	$ 9.07	$ 17.94
Unrecognized compensation cost related to stock options	$ 34.1		$ 34.1
Unvested restricted stock outstanding	72,611		72,611		109,073
Stock options outstanding	5,315,496		5,315,496		5,037,663
Vesting description			The performance-based restricted stock units and performance-based stock options vest upon obtaining certain scientific, financial and regulatory milestones through 2020
Potential dilutive securities excluded from computation of diluted net loss per common share			5,388,000	5,142,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period of unrecognized compensation costs			1 year 4 months 24 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period of unrecognized compensation costs			2 years 7 months 6 days
Performance Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock outstanding	71,875		71,875
Potential dilutive securities excluded from computation of diluted net loss per common share			71,875
Performance Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options outstanding	213,750		213,750
Potential dilutive securities excluded from computation of diluted net loss per common share			188,750