Earnings Per Share - Reconciles Numerators and Denominators of Basic Earnings Per Share and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator for Earnings per Common Share-Basic:
Net Income									$ 57,910	$ 50,539	$ 34,200
Less: Income allocated to participating shares									165	147	126
Net Income Allocated to Common Shareholders	14,497	14,660	14,720	14,033	11,859	12,247	14,103	12,330	57,745	50,392	34,074
Numerator for Earnings per Common Share-Diluted:
Net Income									$ 57,910	$ 50,539	$ 34,200
Denominators:
Weighted Average Common Shares Outstanding-Basic									29,683,103	29,647,231	28,976,619
Add: Dilutive potential common shares									25,621	35,322	32,261
Denominator for Treasury Stock Method-Diluted									29,708,724	29,682,553	29,008,880
Weighted Average Common Shares Outstanding-Basic									29,683,103	29,647,231	28,976,619
Add: Average participating shares outstanding									84,918	86,490	107,274
Denominator for Two-Class Method-Diluted									29,768,021	29,733,721	29,083,893
Earnings per common share-basic (dollars per share)									$ 1.95	$ 1.70	$ 1.18
Earnings per common share-diluted (shares)	$ 0.49	$ 0.49	$ 0.49	$ 0.47	$ 0.40	$ 0.41	$ 0.47	$ 0.41	$ 1.95	$ 1.70	$ 1.18
Exercise price of warrants (dollars per share)	$ 31.53								$ 31.53
Warrants [Member]
Denominators:
Anti-dilutive excluded from dilutive potential common shares									517,012	517,012	517,012
Stock Options [Member]
Denominators:
Anti-dilutive excluded from dilutive potential common shares									419,538	619,418	747,443
Restricted Stock [Member]
Denominators:
Anti-dilutive excluded from dilutive potential common shares									59,297	51,169	75,012