Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 350,533	$ 100,748
Less than Twelve Months, Unrealized Losses	(5,391)	(999)
More than Twelve Months, Fair Value	249,051	379,752
More than Twelve Months, Unrealized Losses	(10,175)	(11,332)
Total, Fair Value	599,584	480,500
Total, Unrealized Losses	(15,566)	(12,331)
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	48,332	0
Less than Twelve Months, Unrealized Losses	(1,786)	0
More than Twelve Months, Fair Value	0	48,504
More than Twelve Months, Unrealized Losses	0	(1,622)
Total, Fair Value	48,332	48,504
Total, Unrealized Losses	(1,786)	(1,622)
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value		0
Less than Twelve Months, Unrealized Losses		0
More than Twelve Months, Fair Value		5,442
More than Twelve Months, Unrealized Losses		(86)
Total, Fair Value		5,442
Total, Unrealized Losses		(86)
Obligations of states and state agencies
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	10,934	0
Less than Twelve Months, Unrealized Losses	(106)	0
More than Twelve Months, Fair Value	0	11,136
More than Twelve Months, Unrealized Losses	0	(24)
Total, Fair Value	10,934	11,136
Total, Unrealized Losses	(106)	(24)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	20,525	13,337
Less than Twelve Months, Unrealized Losses	(651)	(426)
More than Twelve Months, Fair Value	11,053	14,637
More than Twelve Months, Unrealized Losses	(236)	(126)
Total, Fair Value	31,578	27,974
Total, Unrealized Losses	(887)	(552)
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	31,459	13,337
Less than Twelve Months, Unrealized Losses	(757)	(426)
More than Twelve Months, Fair Value	11,053	25,773
More than Twelve Months, Unrealized Losses	(236)	(150)
Total, Fair Value	42,512	39,110
Total, Unrealized Losses	(993)	(576)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	252,728	57,543
Less than Twelve Months, Unrealized Losses	(2,742)	(121)
More than Twelve Months, Fair Value	177,821	244,910
More than Twelve Months, Unrealized Losses	(5,993)	(4,839)
Total, Fair Value	430,549	302,453
Total, Unrealized Losses	(8,735)	(4,960)
Trust preferred securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	2,210
Less than Twelve Months, Unrealized Losses	0	(117)
More than Twelve Months, Fair Value	8,618	12,085
More than Twelve Months, Unrealized Losses	(1,969)	(2,836)
Total, Fair Value	8,618	14,295
Total, Unrealized Losses	(1,969)	(2,953)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	17,878	27,500
Less than Twelve Months, Unrealized Losses	(91)	(294)
More than Twelve Months, Fair Value	37,142	28,269
More than Twelve Months, Unrealized Losses	(750)	(924)
Total, Fair Value	55,020	55,769
Total, Unrealized Losses	(841)	(1,218)
Equity securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	136	158
Less than Twelve Months, Unrealized Losses	(15)	(41)
More than Twelve Months, Fair Value	14,417	14,769
More than Twelve Months, Unrealized Losses	(1,227)	(875)
Total, Fair Value	14,553	14,927
Total, Unrealized Losses	$ (1,242)	$ (916)