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Amount of expense related to loan loss not accounted for as debt securities, with evidence of deterioration of credit quality.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended			12 Months Ended
	Mar. 31, 2021 USD ($) Loan SecurityLoan	Jun. 30, 2020 USD ($) Loan	Mar. 31, 2020 USD ($) SecurityLoan	Dec. 31, 2020 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Period for calculating Loss migration rates of portfolio segments	3 years
Allowance for loan losses	$ 26,133,000			$ 25,028,000
Provision for loan losses	$ 830,000		$ 2,126,000
Number of days past due for loans to be considered as nonperforming	90 days
Reasonable period for nonperforming TDRs to be returned to performing status	6 months
Number of days reaching where loans are considered for nonaccrual status	90 days
Conditions where loans are considered for nonaccrual status	A loan may be returned to accruing status only if one of three conditions are met: the loan is well-secured and none of the principal and interest has been past due for a minimum of 90 days; the loan is a TDR and has made a minimum of six months payments; or the principal and interest payments are reasonably assured and a sustained period of performance has occurred, generally six months.
Loans modified as troubled debt restructuring	$ 0		$ 0
Defaulted loans \| SecurityLoan	0		0
Impaired loans	greater than $350
Allowance for loan losses recorded for acquired loans	$ 0			0
Foreclosed assets	0			31,000
Residential Mortgage [Member]
Accounts Notes And Loans Receivable [Line Items]
Residential mortgages in process of foreclosure	371,000			741,000
Commercial and Agriculture [Member]
Accounts Notes And Loans Receivable [Line Items]
Provision for loan losses	(595,000)		$ 180,000
Loans modified as troubled debt restructuring		$ 431,300,000
Number of loans modified \| Loan		813
Commercial and Agriculture [Member] \| Additional 90 Day Modifications [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans modified as troubled debt restructuring		$ 124,400,000
Number of loans modified \| Loan		100
Commercial and Agriculture [Member] \| Second Modifications [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans modified as troubled debt restructuring	$ 70,660,000
Number of loans modified \| Loan	51
TDRs [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ 34,000			$ 35,000