Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 192,382	$ 100,748
Less than Twelve Months, Unrealized Losses	(2,563)	(999)
More than Twelve Months, Fair Value	243,614	379,752
More than Twelve Months, Unrealized Losses	(8,286)	(11,332)
Total, Fair Value	435,996	480,500
Total, Unrealized Losses	(10,849)	(12,331)
U.S. Treasury securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	49,516	0
Less than Twelve Months, Unrealized Losses	(599)	0
More than Twelve Months, Fair Value	0	48,504
More than Twelve Months, Unrealized Losses	0	(1,622)
Total, Fair Value	49,516	48,504
Total, Unrealized Losses	(599)	(1,622)
U.S. government agency securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Unrealized Losses	0	0
More than Twelve Months, Fair Value	4,857	5,442
More than Twelve Months, Unrealized Losses	(4)	(86)
Total, Fair Value	4,857	5,442
Total, Unrealized Losses	(4)	(86)
Obligations of states and state agencies
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		11,136
More than Twelve Months, Unrealized Losses		(24)
Total, Fair Value		11,136
Total, Unrealized Losses		(24)
Municipal bonds
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	19,764	13,337
Less than Twelve Months, Unrealized Losses	(232)	(426)
More than Twelve Months, Fair Value	10,721	14,637
More than Twelve Months, Unrealized Losses	(576)	(126)
Total, Fair Value	30,485	27,974
Total, Unrealized Losses	(808)	(552)
Obligations of states and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	19,764	13,337
Less than Twelve Months, Unrealized Losses	(232)	(426)
More than Twelve Months, Fair Value	10,721	25,773
More than Twelve Months, Unrealized Losses	(576)	(150)
Total, Fair Value	30,485	39,110
Total, Unrealized Losses	(808)	(576)
Residential mortgage-backed securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	107,912	57,543
Less than Twelve Months, Unrealized Losses	(1,601)	(121)
More than Twelve Months, Fair Value	168,049	244,910
More than Twelve Months, Unrealized Losses	(3,796)	(4,839)
Total, Fair Value	275,961	302,453
Total, Unrealized Losses	(5,397)	(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	8,593	12,085
More than Twelve Months, Unrealized Losses	(1,937)	(2,836)
Total, Fair Value	8,593	14,295
Total, Unrealized Losses	(1,937)	(2,953)
Corporate and other debt securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	15,190	27,500
Less than Twelve Months, Unrealized Losses	(131)	(294)
More than Twelve Months, Fair Value	36,908	28,269
More than Twelve Months, Unrealized Losses	(815)	(924)
Total, Fair Value	52,098	55,769
Total, Unrealized Losses	(946)	(1,218)
Equity securities
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	0	158
Less than Twelve Months, Unrealized Losses	0	(41)
More than Twelve Months, Fair Value	14,486	14,769
More than Twelve Months, Unrealized Losses	(1,158)	(875)
Total, Fair Value	14,486	14,927
Total, Unrealized Losses	$ (1,158)	$ (916)