Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 421,553	$ 112,284
Less than Twelve Months, Unrealized Losses	(6,426)	(613)
More than Twelve Months, Fair Value	239,165	390,639
More than Twelve Months, Unrealized Losses	(19,291)	(18,701)
Total, Fair Value	660,718	502,923
Total, Unrealized Losses	(25,717)	(19,314)
Obligations of states and state agencies
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	54,550	4,927
Less than Twelve Months, Unrealized Losses	(1,033)	(50)
More than Twelve Months, Fair Value	1,888	19,050
More than Twelve Months, Unrealized Losses	(81)	(362)
Total, Fair Value	56,438	23,977
Total, Unrealized Losses	(1,114)	(412)
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	44,292	0
Less than Twelve Months, Unrealized Losses	(773)	0
More than Twelve Months, Fair Value	10,034	28,815
More than Twelve Months, Unrealized Losses	(210)	(278)
Total, Fair Value	54,326	28,815
Total, Unrealized Losses	(983)	(278)
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	98,842	4,927
Less than Twelve Months, Unrealized Losses	(1,806)	(50)
More than Twelve Months, Fair Value	11,922	47,865
More than Twelve Months, Unrealized Losses	(291)	(640)
Total, Fair Value	110,764	52,792
Total, Unrealized Losses	(2,097)	(690)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	317,973	107,357
Less than Twelve Months, Unrealized Losses	(4,516)	(563)
More than Twelve Months, Fair Value	166,958	276,580
More than Twelve Months, Unrealized Losses	(5,863)	(5,681)
Total, Fair Value	484,931	383,937
Total, Unrealized Losses	(10,379)	(6,244)
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	4,738	0
Less than Twelve Months, Unrealized Losses	(104)	0
More than Twelve Months, Fair Value	60,285	66,194
More than Twelve Months, Unrealized Losses	(13,137)	(12,380)
Total, Fair Value	65,023	66,194
Total, Unrealized Losses	$ (13,241)	$ (12,380)