Loan and Lease Finance Receivables and Allowance for Credit Losses - Summary of Components of Loan and Lease Finance Receivables (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 558,255	$ 573,571
Construction	56,764	61,300
Commercial real estate	2,156,267	2,169,535
SFR mortgage	163,343	160,703
Consumer	50,083	53,894
Municipal lease finance receivables	109,727	105,767
Auto and equipment leases, net of unearned discount	12,422	12,716
Dairy and livestock	278,502	327,579
Agribusiness	11,240	14,732
Gross loans	3,396,603	3,479,797
Purchase accounting discount	(20,908)	(25,344)
Deferred loan fees, net	(7,487)	(6,925)
Gross loans, net of deferred loan fees and discount	3,189,514	3,252,313
Less: Allowance for credit losses	(92,218)	(92,441)	(91,922)	(93,964)
Net loans	3,275,990	3,355,087
Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	532,941	547,422
Construction	55,703	59,721
Commercial real estate	1,996,198	1,990,107
SFR mortgage	162,022	159,288
Consumer	44,116	47,557
Municipal lease finance receivables	109,727	105,767
Auto and equipment leases, net of unearned discount	12,422	12,716
Dairy and livestock	278,502	327,579
Agribusiness	5,370	9,081
Gross loans	3,197,001	3,259,238
Deferred loan fees, net	(7,487)	(6,925)
Gross loans, net of deferred loan fees and discount	3,189,514	3,252,313
Less: Allowance for credit losses	(92,218)	(92,441)
Net loans	3,097,296	3,159,872
Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	25,314	26,149
Construction	1,061	1,579
Commercial real estate	160,069	179,428
SFR mortgage	1,321	1,415
Consumer	5,967	6,337
Agribusiness	5,870	5,651
Gross loans	199,602	220,559
Purchase accounting discount	(20,908)	(25,344)
Gross loans, net of deferred loan fees and discount	178,694	195,215
Net loans	$ 178,694	$ 195,215