Loans and Lease Finance Receivables and Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract	Sep. 30, 2017 USD ($) Contract SecurityLoan	Sep. 30, 2016 USD ($) Contract	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total fixed rate loans held	$ 2,160,000,000		$ 2,160,000,000
Secure borrowings and available lines of credit from FHLB and Federal Reserve Bank	3,610,000,000		3,610,000,000		$ 3,110,000,000
Loans held-for-sale	0		0		0
Impaired, at carrying value	17,308,000	$ 35,684,000	17,308,000	$ 35,684,000	26,385,000
Loans classified as troubled debt restructured	10,000,000		10,000,000
Impaired, at carrying value	88,000	548,000	88,000	548,000	141,000
Provision for unfunded loan commitments	0	$ 0	0	$ 0
Reserve for credit risk for undisbursed commitments	6,700,000		6,700,000		6,700,000
Allocation of allowance to troubled debt restructuring	$ 5,000		$ 5,000		141,000
Number of loans modified as TDRs | Contract	0	2	3	5
Valley Business Bank [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	$ 4,500,000		$ 4,500,000
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	70.66%		70.66%
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	77.42%		77.42%
Dairy & Livestock and Agribusiness [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 829,000		$ 829,000
Commercial real estate loan outstanding balance modified as a TDR within the previous 12 months that subsequently defaulted			$ 78,000
Number of loans modified as a TDR within the previous 12 months that subsequently defaulted | Contract			1
Nonaccrual Small Business Administration Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,600,000		$ 1,600,000
Impaired Loans Modified in Troubled Debt Restructure [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	10,000,000		10,000,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial real estate loan outstanding balance modified as a TDR within the previous 12 months that subsequently defaulted			$ 3,100,000
Number of loans modified as a TDR within the previous 12 months that subsequently defaulted | Contract			1
Nonaccrual Single Family Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,300,000		$ 1,300,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	6,700,000		6,700,000
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	743,000		743,000
Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	4,300,000		4,300,000
Loans classified as troubled debt restructured	4,300,000		4,300,000
Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	5,700,000		$ 5,700,000
Number of performing loans | SecurityLoan			21
Performing Financing Receivable [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	1,400,000		$ 1,400,000
Number of loans | SecurityLoan			3
Performing Financing Receivable [Member] | Single-family Residential Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	3,200,000		$ 3,200,000
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	432,000		$ 432,000
Number of loans | SecurityLoan			5
Performing Financing Receivable [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	662,000		$ 662,000
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	0		$ 0
Number of loans | SecurityLoan			0
Allowance Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	88,000		$ 88,000
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial real estate loans	3,332,517,000		3,332,517,000		2,930,141,000
Dairy & livestock and agribusiness	270,482,000		270,482,000		338,631,000
Outstanding balance	11,573,000		11,573,000		7,152,000
Loans, Excluding PCI Loans [Member] | Dairy & Livestock Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dairy & livestock and agribusiness	235,200,000		235,200,000		317,900,000
Loans, Excluding PCI Loans [Member] | Agribusiness Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Dairy & livestock and agribusiness	35,300,000		35,300,000		20,700,000
Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	829,000		829,000		0
Loans, Excluding PCI Loans [Member] | Nonaccrual Commercial and Industrial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	313,000		313,000
Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	313,000		313,000		156,000
Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,611,000		1,611,000		2,737,000
Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 743,000		$ 743,000		$ 369,000
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)	5.41%		5.41%		6.16%
Commercial real estate loans	$ 180,200,000		$ 180,200,000		$ 180,600,000
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	102,000,000		102,000,000		127,100,000
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	$ 78,200,000		$ 78,200,000		$ 53,600,000