Stock-Based Compensation - SAR Activity (Detail) - Stock Appreciation Rights [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Stock Appreciation Rights Activity [Line Items]
SARs Outstanding, Beginning Balance	137,725	262,077	515,830
Granted, SARs	18,739		15,585
Exercised, SARs	(17,406)	(124,352)	(263,626)
Canceled, SARs			(5,712)
Shares Outstanding, SARs, Ending Balance	139,058	137,725	262,077
Outstanding, Weighted Average Grant Price Per Share, Beginning Balance	$ 19.57	$ 13.13	$ 13.98
Vested, SARs	139,058
Granted, Weighted Average Grant Price Per Share	$ 70.75		41.19
Exercisable, SARs	115,191
Exercised, Weighted Average Grant Price Per Share	$ 16.13	11.09	13.86
Canceled, Weighted Average Grant Price Per Share			21.94
Outstanding, Weighted Average Grant Price Per Share, Ending Balance	26.89	$ 19.57	$ 13.13
Vested, Weighted Average Grant Price Per Share	26.89
Exercisable, Weighted Average Grant Price Per Share	$ 19.12
Weighted Average Remaining Contractual Life, Outstanding	5 years 3 months 19 days
Weighted Average Remaining Contractual Life, Vested	5 years 3 months 19 days
Weighted Average Remaining Contractual Life, Exercisable	4 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding	$ 11,333,033
Aggregate Intrinsic Value, Vested	11,333,033
Aggregate Intrinsic Value, Exercisable	$ 10,282,908