Share-Based Payments - Summary of Vesting Information for RSU Awards Granted (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units
Share-based compensation Arrangement
Number of units vested under compound annualized TSR during performance period	209,654	354,058	260,000
10% to Less Than 15%
Share-based compensation Arrangement
Number of units vested under compound annualized TSR during performance period			150,000
15% to Less Than 40%
Share-based compensation Arrangement
Number of units vested under compound annualized TSR during performance period			250,000
40% or More | Restricted Stock Units
Share-based compensation Arrangement
Number of units vested under compound annualized TSR during performance period			260,000
8 Percentage Points Below the Russell 2000 Index TSR | Restricted Stock Units
Share-based compensation Arrangement
Percentage of TSR target share amount	50.00%
Equal to the Russell 2000 Index TSR | Restricted Stock Units
Share-based compensation Arrangement
Percentage of TSR target share amount	100.00%
8 or More Percentage Points Above the Russell 2000 Index TSR | Restricted Stock Units
Share-based compensation Arrangement
Percentage of TSR target share amount	150.00%