Share-Based Compensation - Schedule of Restricted Stock Units to be Settled in Cash (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning balance of Outstanding, Restricted Stock Units to be Settled in Cash	471,728	1,192,078	1,575,050
Restricted Stock Units to be Settled in Cash, Granted			461,428
Restricted Stock Units to be Settled in Cash, Vested	(214,192)	(650,729)	(610,680)
Restricted Stock Units to be Settled in Cash, Forfeited	(44,742)	(69,621)	(233,720)
Ending balance of Outstanding, Restricted Stock Units to be Settled in Cash	212,794	471,728	1,192,078