Loans - Contractual Maturity Schedule by Major Category of Loans Excluding Fair Value Adjustments (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	$ 1,306,425
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	175,643
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	189,780
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	94,361
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	588,937
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	189,431
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	52,519
Other Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	15,754
Major Category Of Receivables Due Within One Year [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	151,196
Major Category Of Receivables Due Within One Year [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	5,810
Major Category Of Receivables Due Within One Year [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	5,612
Major Category Of Receivables Due Within One Year [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	53,083
Major Category Of Receivables Due Within One Year [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	21,532
Major Category Of Receivables Due Within One Year [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	61,416
Major Category Of Receivables Due Within One Year [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	1,520
Major Category Of Receivables Due Within One Year [Member] | Other Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	2,223
Major Category Of Receivables Due Within One Year to Five Years [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	518,858
Major Category Of Receivables Due Within One Year to Five Years [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	31,538
Major Category Of Receivables Due Within One Year to Five Years [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	4,045
Major Category Of Receivables Due Within One Year to Five Years [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	27,835
Major Category Of Receivables Due Within One Year to Five Years [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	307,834
Major Category Of Receivables Due Within One Year to Five Years [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	109,373
Major Category Of Receivables Due Within One Year to Five Years [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	38,099
Major Category Of Receivables Due Within One Year to Five Years [Member] | Other Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	134
Major Category Of Receivables Over Five Years [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	636,371
Major Category Of Receivables Over Five Years [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	138,295
Major Category Of Receivables Over Five Years [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	180,123
Major Category Of Receivables Over Five Years [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	13,443
Major Category Of Receivables Over Five Years [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	259,571
Major Category Of Receivables Over Five Years [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	18,642
Major Category Of Receivables Over Five Years [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	12,900
Major Category Of Receivables Over Five Years [Member] | Other Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans excluding fair value adjustments	$ 13,397