Loans and Lease Finance Receivables and Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2019 USD ($) SecurityLoan Contract	Sep. 30, 2018 USD ($) Contract	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total fixed rate loans held	$ 3,830,000,000		$ 3,830,000,000
Secure borrowings and available lines of credit from FHLB and Federal Reserve Bank	5,910,000,000		5,910,000,000		$ 5,710,000,000
Impaired, at carrying value	9,780,000	$ 20,183,000	9,780,000	$ 20,183,000	23,545,000
Impaired, at carrying value	540,000	$ 83,000	$ 540,000	$ 83,000	561,000
Number of performing loans | SecurityLoan			12
Reserve for credit risk for undisbursed commitments	9,000,000.0		$ 9,000,000.0		9,000,000.0
Allocation of allowance to troubled debt restructuring	$ 0		$ 0		$ 490,000
Number of loans modified as a TDR within the previous 12 months that subsequently defaulted | Contract	0	0	0	0
Percentage of PCI loans to Loan Receivable	0.20%		0.20%		0.20%
Outstanding Loans Held-for-sale	$ 0		$ 0		$ 0
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	77.01%		77.01%
Notes receivable gross	$ 5,375,668,000		$ 5,375,668,000		5,408,636,000
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total gross loan portfolio (excluding PCI loans)	71.69%		71.69%
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	3,400,000		3,400,000
Loans classified as troubled debt restructured	3,400,000		3,400,000
Single-family Residential Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	888,000		888,000
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,100,000		1,100,000
Nonaccrual Single Family Mortgage Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,700,000		2,700,000
Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	249,000		249,000
Loans classified as troubled debt restructured	249,000		249,000
Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired, at carrying value	9,800,000		9,800,000
Loans classified as troubled debt restructured	3,200,000		3,200,000
Performing Financing Receivable [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	542,000		$ 542,000
Number of loans | SecurityLoan			1
Performing Financing Receivable [Member] | Impaired Loans Modified in Troubled Debt Restructure [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	3,200,000		$ 3,200,000
Performing Financing Receivable [Member] | Single-family Residential Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	2,100,000		$ 2,100,000
Number of loans | SecurityLoan			8
Performing Financing Receivable [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	88,000		$ 88,000
Number of loans | SecurityLoan			2
Performing Financing Receivable [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans classified as troubled debt restructured	417,000		$ 417,000
Performing Financing Receivable [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan			1
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable gross	311,200,000		$ 311,200,000
Impaired, at carrying value					23,500,000
Outstanding balance	6,612,000		6,612,000		19,951,000
Loans, Excluding PCI Loans [Member] | Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable gross					5,394,229,000
Loans, Excluding PCI Loans [Member] | Dairy & Livestock Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable gross	251,300,000		251,300,000		340,500,000
Loans, Excluding PCI Loans [Member] | Agribusiness Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable gross	60,000,000.0		60,000,000.0		54,000,000.0
Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	1,550,000		1,550,000		7,490,000
Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	2,706,000		2,706,000		$ 2,892,000
Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding balance	$ 385,000		$ 385,000
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)	4.57%		4.57%		4.27%
Notes receivable gross	$ 245,600,000		$ 245,600,000		$ 231,000,000.0
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]
Notes receivable gross	130,400,000		130,400,000		126,900,000
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]
Notes receivable gross	$ 115,100,000		$ 115,100,000		$ 104,100,000