Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 986,105	$ 813,474
Gross Unrealized Gains	16,025	13,821
Gross Unrealized Losses	(43,474)	(19,479)
Fair Value	958,656	807,816
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	99,839	99,843
Gross Unrealized Gains
Gross Unrealized Losses	(10,053)	(2,218)
Fair Value	89,786	97,625
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	54,917	44,215
Gross Unrealized Gains	1,504	1,547
Gross Unrealized Losses	(613)
Fair Value	55,808	45,762
Obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,608	16,210
Gross Unrealized Gains	791	417
Gross Unrealized Losses	(2,165)
Fair Value	39,234	16,627
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	590,225	506,695
Gross Unrealized Gains	4,414	6,818
Gross Unrealized Losses	(20,812)	(3,359)
Fair Value	573,827	510,154
Trust preferred securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,973	68,931
Gross Unrealized Gains	6,088	240
Gross Unrealized Losses	(5,648)	(11,739)
Fair Value	68,413	57,432
Corporate and other debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,523	28,274
Gross Unrealized Gains	2,008	2,728
Gross Unrealized Losses	(2,285)	(294)
Fair Value	84,246	30,708
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,020	49,306
Gross Unrealized Gains	1,220	2,071
Gross Unrealized Losses	(1,898)	(1,869)
Fair Value	$ 47,342	$ 49,508