Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Shares Under Option
Shares Under Option, Outstanding Beginning Balance	622,945	738,792	1,384,537
Shares Under Option, Granted	0	0	0
Shares Under Option, Exercised	(106,589)	(115,847)	(645,745)
Shares Under Option, Forfeited	(46,000)
Shares Under Option, Expired	0	0	0
Shares Under Option, Outstanding Ending Balance	470,356	622,945	738,792	1,384,537
Shares Under Option, Ending vested and expected to vest	470,356
Shares Under Option, Exercisable	316,356	422,945	162,792
Weighted-Average Exercise Price
Weighted-Average Exercise Price, Outstanding Beginning Balance	$ 18.55	$ 18.39	$ 17.58
Weighted-Average Exercise Price, Granted	$ 0	$ 0	$ 0
Weighted-Average Exercise Price, Exercised	$ 18.48	$ 17.56	$ 16.64
Weighted-Average Exercise Price, Forfeited	$ 18.48
Weighted-Average Exercise Price, Expired	$ 0	$ 0	$ 0
Weighted-Average Exercise Price, Outstanding Ending Balance	$ 18.57	$ 18.55	$ 18.39	$ 17.58
Weighted-Average Exercise Price, Ending vested and expected to vest	$ 18.57
Weighted-Average Exercise Price, Exercisable	$ 18.48	$ 18.48	$ 17.82
Weighted-Average Remaining Contractual Term
Weighted-Average Remaining Contractual Term, Outstanding	3 years 1 month 6 days	4 years 1 month 6 days	4 years 10 months 24 days	4 years 4 months 24 days
Weighted-Average Remaining Contractual Term, Ending vested and expected to vest	3 years 1 month 6 days
Weighted-Average Remaining Contractual Term, Exercisable	3 years 2 months 12 days	4 years 2 months 12 days	4 years
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding Beginning Balance	$ 7,289	$ 8,891	$ 14,587
Aggregate Intrinsic Value, Outstanding Ending balance	5,087	7,289	8,891	14,587
Aggregate Intrinsic Value, Ending vested and expected to vest	5,087
Aggregate Intrinsic Value, Exercisable	$ 4,565	$ 5,460	$ 1,243