Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 250,008	$ 112,284
Less than Twelve Months, Unrealized Losses	(1,418)	(613)
More than Twelve Months, Fair Value	256,099	390,639
More than Twelve Months, Unrealized Losses	(15,106)	(18,701)
Total, Fair Value	506,107	502,923
Total, Unrealized Losses	(16,524)	(19,314)
Obligations of States and State Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	22,194	4,927
Less than Twelve Months, Unrealized Losses	(145)	(50)
More than Twelve Months, Fair Value	1,961	19,050
More than Twelve Months, Unrealized Losses	(7)	(362)
Total, Fair Value	24,155	23,977
Total, Unrealized Losses	(152)	(412)
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	11,876	0
Less than Twelve Months, Unrealized Losses	(79)	0
More than Twelve Months, Fair Value	10,253	28,815
More than Twelve Months, Unrealized Losses	(7)	(278)
Total, Fair Value	22,129	28,815
Total, Unrealized Losses	(86)	(278)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	34,070	4,927
Less than Twelve Months, Unrealized Losses	(224)	(50)
More than Twelve Months, Fair Value	12,214	47,865
More than Twelve Months, Unrealized Losses	(14)	(640)
Total, Fair Value	46,284	52,792
Total, Unrealized Losses	(238)	(690)
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	211,113	107,357
Less than Twelve Months, Unrealized Losses	(1,178)	(563)
More than Twelve Months, Fair Value	177,774	276,580
More than Twelve Months, Unrealized Losses	(2,626)	(5,681)
Total, Fair Value	388,887	383,937
Total, Unrealized Losses	(3,804)	(6,244)
Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	4,825	0
Less than Twelve Months, Unrealized Losses	(16)	0
More than Twelve Months, Fair Value	66,111	66,194
More than Twelve Months, Unrealized Losses	(12,466)	(12,380)
Total, Fair Value	70,936	66,194
Total, Unrealized Losses	$ (12,482)	$ (12,380)