Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 666,754	$ 124,360
Less than Twelve Months, Unrealized Losses	(29,049)	(760)
More than Twelve Months, Fair Value	52,504	48,802
More than Twelve Months, Unrealized Losses	(12,319)	(14,570)
Total, Fair Value	719,258	173,162
Total, Unrealized Losses	(41,368)	(15,330)
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	130,784	15,518
Less than Twelve Months, Unrealized Losses	(9,564)	(197)
More than Twelve Months, Fair Value	1,345
More than Twelve Months, Unrealized Losses	(96)
Total, Fair Value	132,129	15,518
Total, Unrealized Losses	(9,660)	(197)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	520,005	80,152
Less than Twelve Months, Unrealized Losses	(19,225)	(355)
Total, Fair Value	520,005	80,152
Total, Unrealized Losses	(19,225)	(355)
Trust preferred securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	15,916	28,690
Less than Twelve Months, Unrealized Losses	(259)	(208)
More than Twelve Months, Fair Value	51,159	48,802
More than Twelve Months, Unrealized Losses	(12,223)	(14,570)
Total, Fair Value	67,075	77,492
Total, Unrealized Losses	(12,482)	(14,778)
Corporate and other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	49
Less than Twelve Months, Unrealized Losses	(1)
Total, Fair Value	49
Total, Unrealized Losses	$ (1)