Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 176,827	$ 124,360
Less than Twelve Months, Unrealized Losses	(2,080)	(760)
More than Twelve Months, Fair Value	50,112	48,802
More than Twelve Months, Unrealized Losses	(13,265)	(14,570)
Total, Fair Value	226,939	173,162
Total, Unrealized Losses	(15,345)	(15,330)
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	84,770	15,518
Less than Twelve Months, Unrealized Losses	(1,167)	(197)
Total, Fair Value	84,770	15,518
Total, Unrealized Losses	(1,167)	(197)
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	85,977	80,152
Less than Twelve Months, Unrealized Losses	(817)	(355)
Total, Fair Value	85,977	80,152
Total, Unrealized Losses	(817)	(355)
Trust preferred securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	6,080	28,690
Less than Twelve Months, Unrealized Losses	(96)	(208)
More than Twelve Months, Fair Value	50,112	48,802
More than Twelve Months, Unrealized Losses	(13,265)	(14,570)
Total, Fair Value	56,192	77,492
Total, Unrealized Losses	$ (13,361)	$ (14,778)