Loan and Lease Finance Receivables and Allowance for Credit Losses - Additional Information (Detail) (USD $)	3 Months Ended					12 Months Ended					3 Months Ended			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Nonaccrual Commercial Real Estate Loans [Member]	Mar. 31, 2013 Nonaccrual Consumer Loans [Member]	Dec. 31, 2012 Non-Covered Impaired Loans [Member]	Mar. 31, 2013 Performing Financing Receivable [Member] SecurityLoan	Mar. 31, 2013 Nonaccrual Single Family Mortgage Loans [Member]	Mar. 31, 2013 Non-Performing Financing Receivable [Member]	Mar. 31, 2013 Allowance Impaired Loans [Member]	Dec. 31, 2012 Allowance Impaired Non-Covered Loans [Member]	Mar. 31, 2013 Commercial Real Estate Loans [Member] SecurityLoan	Mar. 31, 2013 Nonaccrual Dairy and Livestock Loans [Member]	Mar. 31, 2013 Non-covered Impaired Loans Modified in Troubled Debt Restructure [Member]	Mar. 31, 2013 Dairy and livestock [Member] SecurityLoan	Mar. 31, 2013 Single-family residential loans [Member] SecurityLoan	Mar. 31, 2013 Commercial and Industrial [Member] SecurityLoan	Mar. 31, 2013 Construction [Member] SecurityLoan	Mar. 31, 2013 Nonaccrual Residential Construction and Land Loans [Member]	Mar. 31, 2013 Nonaccrual Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of gross loan portfolio											63.48%						1.67%
Fixed rate loans held	$ 1,560,000,000
Secure borrowings from FHLB and Federal Reserve Bank	2,310,000,000	2,320,000,000
Impaired, at carrying value	112,721,000	108,390,000			108,400,000	112,700,000		29,600,000		2,300,000			87,200,000
Outstanding balance			20,000,000	226,000			11,500,000					9,400,000						10,600,000	3,400,000
Impaired, at carrying value	87,200,000					57,600,000		29,600,000	4,100,000		21,500,000			16,000,000	2,000,000	1,200,000	16,900,000
Number of performing loans						44
Amount of impaired loan transferred to non accrual status	10,600,000
Reserve for unfunded commitments	0
Reserve for credit risk for undisbursed commitments	8,600,000	8,600,000
Number of loans											14			13	7	8	2
Allocation of allowance to troubled debt restructuring	$ 1,800,000	$ 1,400,000