Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 677,371		$ 598,837
Gross Unrealized Gains	16,584		17,428
Gross Unrealized Losses	(20,044)		(49,745)
Fair Value	673,911		566,520
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,148		89,787
Gross Unrealized Gains	1,465		1,204
Gross Unrealized Losses	(36)		(243)
Fair Value	47,577		90,748
Obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,772		18,893
Gross Unrealized Gains	229		1,322
Gross Unrealized Losses	(529)		(1)
Fair Value	16,472		20,214
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	463,489		304,631
Gross Unrealized Gains	10,460		10,950
Gross Unrealized Losses	(3,222)		(5,444)
Fair Value	470,727		310,137
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	72,336	[1]	106,931	[1]
Gross Unrealized Gains	623	[1]	78	[1]
Gross Unrealized Losses	(14,364)	[1]	(36,585)	[1]
Fair Value	58,595	[1]	70,424	[1]
Corporate and other debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,320		30,663
Gross Unrealized Gains	3,076		2,554
Gross Unrealized Losses	(9)		(173)
Fair Value	31,387		33,044
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,306		47,932
Gross Unrealized Gains	731		1,320
Gross Unrealized Losses	(1,884)		(7,299)
Fair Value	$ 49,153		$ 41,953

[1] Includes three pooled trust preferred securities, principally collateralized by securities issued by banks and insurance companies.