Income Taxes - Schedule of Deferred Tax Assets and Liabilities - (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred federal income tax assets:
Allowance for credit losses	$ 17,237	$ 6,475
Allowance for unfunded commitments	1,123
Interest on nonaccrual loans	1,003	344
Postretirement benefit costs	546	613
Deferred compensation	2,057	1,208
Individually evaluated loans	2,483	855
Deferred loan origination fees and costs		474
Accrued vacation	10	11
Accrued bonus	1,065	938
Right of use asset	3,731	1,993
Net operating loss carryforward	307
Other	2,175	1,173
Total deferred federal income tax assets	31,737	14,084
Deferred federal income tax liabilities:
FHLB stock dividends		1,404
Goodwill	4,542	4,956
Mortgage servicing rights	2,762	2,156
Fixed assets	2,230	1,935
Other intangible assets	7,118	647
Loan mark to market		5
Deferred loan origination fees and costs	1,294
Net unrealized gains on available-for-sale securities	4,009	1,286
Prepaid expenses	762	708
Lease liabilities	3,557	1,876
Other	25	16
Total deferred federal income tax liabilities	26,299	14,989
Net deferred federal income tax asset	$ 5,438
Net deferred federal income tax liability		$ (905)