Allowance for Credit Losses - Troubled Debt Restructurings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) investmentSecurity	Dec. 31, 2019 USD ($) investmentSecurity	Dec. 31, 2018 USD ($) investmentSecurity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	18	18	17
Pre-mod outstanding principal balance	$ 6,010	$ 2,851	$ 3,768
Post-mod outstanding principal balance	6,074	2,829	3,755
Financial impact due to TDR taken as additional provision	$ 471	$ 59	$ 781
Number that defaulted during the period | investmentSecurity	8,000	1	6
Recorded investment of TDR's that defaulted during the period	$ 2,862	$ 7	$ 565
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (90)	$ 0	$ 0
Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	10	2	7
Pre-mod outstanding principal balance	$ 3,732	$ 60	$ 1,782
Post-mod outstanding principal balance	3,827	67	1,779
Financial impact due to TDR taken as additional provision	$ 381	$ 0	$ 491
Number that defaulted during the period | investmentSecurity	3,000	0	1
Recorded investment of TDR's that defaulted during the period	$ 1,542	$ 0	$ 169
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	2	6	4
Pre-mod outstanding principal balance	$ 172	$ 873	$ 888
Post-mod outstanding principal balance	169	877	893
Financial impact due to TDR taken as additional provision	$ 0	$ 59	$ (35)
Number that defaulted during the period | investmentSecurity	5,000	0	2
Recorded investment of TDR's that defaulted during the period	$ 1,320	$ 0	$ 248
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (90)	$ 0	$ 0
Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	6	10	6
Pre-mod outstanding principal balance	$ 2,106	$ 1,918	$ 1,098
Post-mod outstanding principal balance	2,078	1,885	1,083
Financial impact due to TDR taken as additional provision	$ 90	$ 0	$ 325
Number that defaulted during the period | investmentSecurity	0	1	3
Recorded investment of TDR's that defaulted during the period	$ 0	$ 7	$ 148
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0
Number that defaulted during the period | investmentSecurity	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Agriculture production
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0
Number that defaulted during the period | investmentSecurity	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Leases
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0
Number that defaulted during the period | investmentSecurity	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
CRE non-owner occupied | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	1	0	1
Pre-mod outstanding principal balance	$ 319	$ 0	$ 39
Post-mod outstanding principal balance	314	0	38
Financial impact due to TDR taken as additional provision	$ 314	$ 0	$ 38
Number that defaulted during the period | investmentSecurity	1,000	0	1
Recorded investment of TDR's that defaulted during the period	$ 141	$ 0	$ 169
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
CRE owner occupied | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	4	2	2
Pre-mod outstanding principal balance	$ 1,847	$ 60	$ 555
Post-mod outstanding principal balance	1,877	67	555
Financial impact due to TDR taken as additional provision	$ 67	$ 0	$ 11
Number that defaulted during the period | investmentSecurity	1,000	0	0
Recorded investment of TDR's that defaulted during the period	$ 950	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Multifamily | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0
Number that defaulted during the period | investmentSecurity	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
Farmland | Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	5	0	4
Pre-mod outstanding principal balance	$ 1,566	$ 0	$ 1,188
Post-mod outstanding principal balance	1,636	0	1,186
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 442
Number that defaulted during the period | investmentSecurity	1,000	0	0
Recorded investment of TDR's that defaulted during the period	$ 451	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
SFR 1-4 1st DT liens | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	3	1
Pre-mod outstanding principal balance	$ 0	$ 659	$ 156
Post-mod outstanding principal balance	0	662	156
Financial impact due to TDR taken as additional provision	$ 0	$ 30
Number that defaulted during the period | investmentSecurity	3,000	0	0
Recorded investment of TDR's that defaulted during the period	$ 1,180	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0
SFR HELOCs and junior liens | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	2	3	3
Pre-mod outstanding principal balance	$ 172	$ 214	$ 732
Post-mod outstanding principal balance	169	215	737
Financial impact due to TDR taken as additional provision	$ 0	$ 29	$ (35)
Number that defaulted during the period | investmentSecurity	2,000	0	2
Recorded investment of TDR's that defaulted during the period	$ 140	$ 0	$ 248
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ (90)	$ 0	$ 0
Other | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number | investmentSecurity	0	0	0
Pre-mod outstanding principal balance	$ 0	$ 0	$ 0
Post-mod outstanding principal balance	0	0	0
Financial impact due to TDR taken as additional provision	$ 0	$ 0	$ 0
Number that defaulted during the period | investmentSecurity	0	0	0
Recorded investment of TDR's that defaulted during the period	$ 0	$ 0	$ 0
Financial impact due to the default of previous TDR taken as charge-offs or additional provisions	$ 0	$ 0	$ 0