Loans and Lease Finance Receivables and Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2015 USD ($) SecurityLoan	Jun. 30, 2016 USD ($) SecurityLoan	Jun. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total fixed rate loans held	$ 2,040,000,000		$ 2,040,000,000
Secure borrowings and available lines of credit from FHLB and Federal Reserve Bank	3,130,000,000		3,130,000,000		$ 2,910,000,000
Impaired, at carrying value	37,764,000	$ 67,390,000	37,764,000	$ 67,390,000	63,706,000
Loans classified as troubled debt restructured	32,300,000		32,300,000
Provision for unfunded loan commitments	0	$ 0	0	$ 500,000
Reserve for credit risk for undisbursed commitments	7,200,000		7,200,000		7,200,000
Allocation of allowance to troubled debt restructuring	$ 609,000		$ 609,000		607,000
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	69.13%		69.13%
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	2.25%		2.25%
Nonaccrual Small Business Administration Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 2,600,000		$ 2,600,000
Impaired Loans Modified in Troubled Debt Restructure [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	32,300,000		32,300,000
Nonaccrual Single Family Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,400,000		2,400,000
Nonaccrual Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	11,400,000		11,400,000
Nonaccrual Consumer Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	428,000		428,000
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial real estate loans	2,884,332,000		2,884,332,000		2,643,184,000
Dairy & livestock and agribusiness	213,830,000		213,830,000		305,509,000
Impaired, at carrying value					63,700,000
Outstanding balance	17,472,000		17,472,000		21,019,000
Loans, Excluding PCI Loans [Member] | Nonaccrual Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	568,000		568,000
Loans, Excluding PCI Loans [Member] | Dairy & Livestock Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dairy & livestock and agribusiness	200,200,000		200,200,000		287,000,000
Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	568,000		568,000		704,000
Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,637,000		2,637,000		2,567,000
Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	428,000		428,000		519,000
Loans, Excluding PCI Loans [Member] | Agribusiness Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dairy & livestock and agribusiness	$ 14,100,000		$ 14,100,000		$ 18,500,000
Loans, Excluding PCI Loans [Member] | Loans Secured by Farmland [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	6.60%		6.60%		6.54%
Commercial real estate loans	$ 190,500,000		$ 190,500,000		$ 173,000,000
Loans, Excluding PCI Loans [Member] | Loans Secured by Farmland [Member] | Loans Secured by Dairy & Livestock Land [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial real estate loans	135,600,000		135,600,000		128,400,000
Loans, Excluding PCI Loans [Member] | Loans Secured by Farmland [Member] | Loans Secured by Agricultural Land [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial real estate loans	54,800,000		54,800,000		44,600,000
Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	12,000,000		12,000,000
Loans classified as troubled debt restructured	12,000,000		12,000,000
Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	37,800,000		37,800,000
Loans classified as troubled debt restructured	20,300,000		$ 20,300,000
Number of performing loans | SecurityLoan			31
Performing Financing Receivable [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	6,500,000		$ 6,500,000
Number of loans | SecurityLoan			9
Performing Financing Receivable [Member] | Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	7,700,000		$ 7,700,000
Number of loans | SecurityLoan			1
Performing Financing Receivable [Member] | Dairy & Livestock Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	697,000		$ 697,000
Number of loans | SecurityLoan			1
Performing Financing Receivable [Member] | Single-family Residential Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	3,300,000		$ 3,300,000
Number of loans | SecurityLoan			11
Performing Financing Receivable [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	879,000		$ 879,000
Number of loans | SecurityLoan			6
Performing Financing Receivable [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	861,000		$ 861,000
Number of loans | SecurityLoan			2
Performing Financing Receivable [Member] | Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	$ 401,000		$ 401,000
Number of loans | SecurityLoan			1
Number of loans removed from TDR classification | SecurityLoan	0	0	0	0
Allowance Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value					$ 669,000
Loans classified as troubled debt restructured	$ 630,000		$ 630,000