Allowance for Credit Losses on Loans - Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) investment_security	Jun. 30, 2019 USD ($) investment_security	Jun. 30, 2020 USD ($) investment_security	Jun. 30, 2019 USD ($) investment_security
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	6	6	10
Pre-mod outstanding principal balance	$ 0	$ 1,847	$ 680	$ 2,145
Post-mod outstanding principal balance	0	1,817	738	2,114
Financial impact due to TDR taken as additional provision	$ 0	$ 29	$ 21	$ 31
Number that defaulted during the period | investment_security	1	0	2,000	1
Recorded investment of TDR's that defaulted during the period	$ 735	$ 0	$ 1,037	$ 7
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	3	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 487	$ 0
Post-mod outstanding principal balance	0	0	549	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	2	2	4
Pre-mod outstanding principal balance	$ 0	$ 93	$ 172	$ 377
Post-mod outstanding principal balance	0	95	169	377
Financial impact due to TDR taken as additional provision	$ 0	$ 27	$ 0	$ 0
Number that defaulted during the period | investment_security	1		2,000	0
Recorded investment of TDR's that defaulted during the period	$ 735		$ 1,037	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	4	1	6
Pre-mod outstanding principal balance	$ 0	$ 1,754	$ 21	$ 1,768
Post-mod outstanding principal balance	0	1,722	20	1,737
Financial impact due to TDR taken as additional provision	$ 0	$ 2	$ 21	$ 31
Number that defaulted during the period | investment_security	0	0	0	1
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 7
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Agriculture production
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Leases
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
CRE non-owner occupied | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	1	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 257	$ 0
Post-mod outstanding principal balance	0	0	251	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
CRE owner occupied | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Multifamily | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Farmland | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	2	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 230	$ 0
Post-mod outstanding principal balance	0	0	298	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
SFR 1-4 1st DT Liens | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	1
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 163
Post-mod outstanding principal balance	0	0	0	162
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	1	0	2,000	0
Recorded investment of TDR's that defaulted during the period	$ 735	$ 0	$ 1,037	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
SFR HELOCs and junior liens | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	2	2	3
Pre-mod outstanding principal balance	$ 0	$ 93	$ 172	$ 214
Post-mod outstanding principal balance	0	95	169	215
Financial impact due to TDR taken as additional provision	$ 0	$ 27	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0
Other | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investment_security	0	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0	$ 0
Number that defaulted during the period | investment_security	0	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0	$ 0