Loans and Lease Finance Receivables and Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014 SecurityLoan	Dec. 31, 2013 SecurityLoan	Dec. 31, 2012	Dec. 31, 2014 SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Fixed rate loans held	$ 1,790,000,000			$ 1,790,000,000
Secure borrowings from FHLB and Federal Reserve Bank	2,780,000,000	2,310,000,000		2,780,000,000
Impaired construction speculative loans	85,792,000	106,909,000	108,390,000	85,792,000
Loans classified as troubled debt restructured	73,900,000			73,900,000
Provision for unfunded loan commitments	(1,250,000)	500,000	(1,000,000)
Reserve for credit risk for undisbursed commitments	7,800,000	9,100,000		7,800,000
Number of loans	0	0
Allocation of allowance to troubled debt restructuring	726,000	2,700,000		726,000
Nonaccrual commercial real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	23,300,000			23,300,000
Nonaccrual consumer loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	736,000			736,000
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans		106,900,000
Outstanding balance	32,186,000	39,954,000		32,186,000
Nonaccrual commercial and industrial loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,300,000			2,300,000
Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,500,000			2,500,000
Small Business Administration [Member] | Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,481,000	1,686,000		2,481,000
Construction Speculative [Member] | Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance		9,966,000
Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans	85,800,000			85,800,000
Loans classified as troubled debt restructured	53,600,000			53,600,000
Number of performing loans				36
Number of loans removed from TDR classification	0	0
Performing financing receivable [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	699,000			699,000
Number of loans	1
Nonperforming financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans	20,300,000			20,300,000
Loans classified as troubled debt restructured	20,300,000			20,300,000
Nonperforming financing receivable [Member] | Construction Speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans	9,600,000			9,600,000
Nonaccrual single family mortgage loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	3,200,000			3,200,000
Allowance impaired loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	1,500,000			1,500,000
Allowance impaired non-covered loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans		3,200,000
Commercial real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio	67.25%			67.25%
Number of loans			1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			2,400,000
Commercial real estate loans [Member] | Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	24,700,000			24,700,000
Number of loans	11
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loan portfolio	1.44%			1.44%
Number of loans			1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			10,700,000
Construction [Member] | Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	7,700,000			7,700,000
Number of loans	1
Dairy & livestock and agribusiness [Member] | Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	15,700,000			15,700,000
Number of loans	8
Single-family Residential Loans [Member] | Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	3,700,000			3,700,000
Number of loans	11
Commercial and industrial [Member] | Performing financing receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	711,000			711,000
Number of loans	3
Nonaccrual dairy and livestock loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	103,000			103,000
Impaired loans modified in troubled debt restructure [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired construction speculative loans	$ 73,900,000			$ 73,900,000