Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	$ 119,881	$ 100,748
Less than Twelve Months, Unrealized Losses	(1,370)	(999)
More than Twelve Months, Fair Value	283,296	379,752
More than Twelve Months, Unrealized Losses	(7,677)	(11,332)
Total, Fair Value	403,177	480,500
Total, Unrealized Losses	(9,047)	(12,331)
U.S. Treasury Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	49,520	0
Less than Twelve Months, Unrealized Losses	(602)	0
More than Twelve Months, Fair Value	0	48,504
More than Twelve Months, Unrealized Losses	0	(1,622)
Total, Fair Value	49,520	48,504
Total, Unrealized Losses	(602)	(1,622)
U.S. Government Agency Securities [Member]
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	5,293	5,442
More than Twelve Months, Unrealized Losses	(94)	(86)
Total, Fair Value	5,293	5,442
Total, Unrealized Losses	(94)	(86)
Obligations of States and State Agencies [Member]
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 [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	19,072	13,337
Less than Twelve Months, Unrealized Losses	(204)	(426)
More than Twelve Months, Fair Value	11,044	14,637
More than Twelve Months, Unrealized Losses	(236)	(126)
Total, Fair Value	30,116	27,974
Total, Unrealized Losses	(440)	(552)
Obligations of States and Political Subdivisions [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	19,072	13,337
Less than Twelve Months, Unrealized Losses	(204)	(426)
More than Twelve Months, Fair Value	11,044	25,773
More than Twelve Months, Unrealized Losses	(236)	(150)
Total, Fair Value	30,116	39,110
Total, Unrealized Losses	(440)	(576)
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	32,256	57,543
Less than Twelve Months, Unrealized Losses	(518)	(121)
More than Twelve Months, Fair Value	216,266	244,910
More than Twelve Months, Unrealized Losses	(3,728)	(4,839)
Total, Fair Value	248,522	302,453
Total, Unrealized Losses	(4,246)	(4,960)
Trust Preferred Securities [Member]
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	8,538	12,085
More than Twelve Months, Unrealized Losses	(2,103)	(2,836)
Total, Fair Value	8,538	14,295
Total, Unrealized Losses	(2,103)	(2,953)
Corporate and Other Debt Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	18,987	27,500
Less than Twelve Months, Unrealized Losses	(43)	(294)
More than Twelve Months, Fair Value	27,622	28,269
More than Twelve Months, Unrealized Losses	(408)	(924)
Total, Fair Value	46,609	55,769
Total, Unrealized Losses	(451)	(1,218)
Equity Securities [Member]
Schedule of Held-to-Maturity Securities [Line Items]
Less than Twelve Months, Fair Value	46	158
Less than Twelve Months, Unrealized Losses	(3)	(41)
More than Twelve Months, Fair Value	14,533	14,769
More than Twelve Months, Unrealized Losses	(1,108)	(875)
Total, Fair Value	14,579	14,927
Total, Unrealized Losses	$ (1,111)	$ (916)