Loans - Loans Individually Evaluated for Impairment by Portfolio Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment	$ 9,388	$ 9,360	$ 10,125	$ 9,388	$ 10,125	$ 9,360
Related Allowance	2,400	1,657	1,198	2,400	1,198	1,657
Recorded Investment	11,849	14,532	15,259	11,849	15,259	14,532
Average Recorded Investment	11,639	14,868	13,517	12,360	8,613	10,234
With no related allowance recorded, Recorded Investment	2,461	5,172	5,134	2,461	5,134	5,172
Consumer Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment		202	205		205	202
With a specific allowance recorded, Unpaid Principal Balance		202	205		205	202
Related Allowance		31	43		43	31
With a specific allowance recorded, Average Recorded Investment			119		60	126
With a specific allowance recorded, Interest Income Recognized			8		10
With a specific allowance recorded, Interest Income Recognized Cash Basis						3
Recorded Investment	714	1,001	985	714	985	1,001
Unpaid Principal Balance	714	1,001	985	714	985	1,001
Average Recorded Investment	453		910	485	748	864
Interest Income Recognized	1		49	4	130
Interest Income Recognized						22
Interest Income Recognized Cash Basis	3		5	10	9	13
With no related allowance recorded, Recorded Investment	714	799	780	714	780	799
With no related allowance recorded, Unpaid Principal Balance	714	799	780	714	780	799
With no related allowance recorded, Average Recorded Investment	453		791	485	688	738
With no related allowance recorded, Interest Income Recognized	1		41	4	120
With no related allowance recorded, Interest Income Recognized						22
With no related allowance recorded, Interest Income Recognized Cash Basis	3		5	10	9	10
Agricultural Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment	4,844	5,210	5,425	4,844	5,425	5,210
With a specific allowance recorded, Unpaid Principal Balance	4,844	5,210	5,425	4,844	5,425	5,210
Related Allowance	636	600	432	636	432	600
With a specific allowance recorded, Average Recorded Investment	5,104		5,450	5,273	2,475	3,175
With a specific allowance recorded, Interest Income Recognized	4		2	15	7	6
With a specific allowance recorded, Interest Income Recognized Cash Basis						102
Recorded Investment	6,051	6,756	7,014	6,051	7,014	6,756
Unpaid Principal Balance	6,051	6,759	7,014	6,051	7,014	6,759
Average Recorded Investment	6,151		6,323	6,464	2,926	3,904
Interest Income Recognized	23		4	68	13	24
Interest Income Recognized Cash Basis						114
With no related allowance recorded, Recorded Investment	1,207	1,546	1,589	1,207	1,589	1,546
With no related allowance recorded, Unpaid Principal Balance	1,207	1,549	1,589	1,207	1,589	1,549
With no related allowance recorded, Average Recorded Investment	1,047		873	1,191	451	729
With no related allowance recorded, Interest Income Recognized	19		2	53	6	18
With no related allowance recorded, Interest Income Recognized Cash Basis						12
Agricultural [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment	117	176	188	117	188	176
With a specific allowance recorded, Unpaid Principal Balance	117	176	188	117	188	176
Related Allowance	54	116	28	54	28	116
With a specific allowance recorded, Average Recorded Investment	117		342	123	208	188
With a specific allowance recorded, Interest Income Recognized			3	4	6
Recorded Investment	247	467	721	247	721	467
Unpaid Principal Balance	247	467	721	247	721	467
Average Recorded Investment	247		717	284	575	580
Interest Income Recognized	4		7	12	15
Interest Income Recognized						3
Interest Income Recognized Cash Basis			1		1	3
With no related allowance recorded, Recorded Investment	130	291	533	130	533	291
With no related allowance recorded, Unpaid Principal Balance	130	291	533	130	533	291
With no related allowance recorded, Average Recorded Investment	130		375	161	367	392
With no related allowance recorded, Interest Income Recognized	4		4	8	9
With no related allowance recorded, Interest Income Recognized						3
With no related allowance recorded, Interest Income Recognized Cash Basis			1		1	3
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment	3,427	2,765	3,033	3,427	3,033	2,765
With a specific allowance recorded, Unpaid Principal Balance	3,427	2,765	3,033	3,427	3,033	2,765
Related Allowance	710	20	44	710	44	20
With a specific allowance recorded, Average Recorded Investment	1,395		3,046	1,800	2,389	2,524
With a specific allowance recorded, Interest Income Recognized	31			58		128
With a specific allowance recorded, Interest Income Recognized Cash Basis				3
Recorded Investment	3,607	2,950	3,219	3,607	3,219	2,950
Unpaid Principal Balance	3,607	2,950	3,219	3,607	3,219	2,950
Average Recorded Investment	3,914		3,232	3,568	2,588	2,719
Interest Income Recognized	35		1	96	3	141
Interest Income Recognized Cash Basis	1			10
With no related allowance recorded, Recorded Investment	180	185	186	180	186	185
With no related allowance recorded, Unpaid Principal Balance	180	185	186	180	186	185
With no related allowance recorded, Average Recorded Investment	2,519		186	1,768	199	195
With no related allowance recorded, Interest Income Recognized	4		1	38	3	13
With no related allowance recorded, Interest Income Recognized Cash Basis	1			7
Commercial and Industrial [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Recorded Investment	1,000	1,007	1,274	1,000	1,274	1,007
With a specific allowance recorded, Unpaid Principal Balance	1,000	1,007	1,274	1,000	1,274	1,007
Related Allowance	1,000	890	651	1,000	651	890
With a specific allowance recorded, Average Recorded Investment	333		1,119	334	929	916
With a specific allowance recorded, Interest Income Recognized	2		3	2	10	52
Recorded Investment	1,215	3,335	3,295	1,215	3,295	3,335
Unpaid Principal Balance	1,215	3,335	3,295	1,215	3,295	3,335
Average Recorded Investment	858		2,309	1,540	1,748	2,138
Interest Income Recognized	2		8	26	28	78
Interest Income Recognized Cash Basis	1			4		5
With no related allowance recorded, Recorded Investment	215	2,328	2,021	215	2,021	2,328
With no related allowance recorded, Unpaid Principal Balance	215	2,328	2,021	215	2,021	2,328
With no related allowance recorded, Average Recorded Investment	525		1,190	1,206	819	1,222
With no related allowance recorded, Interest Income Recognized			5	24	18	26
With no related allowance recorded, Interest Income Recognized Cash Basis	1			4		5
Consumer [Member]
Financing Receivable, Impaired [Line Items]
With a specific allowance recorded, Average Recorded Investment					7	11
With a specific allowance recorded, Interest Income Recognized						1
Recorded Investment	15	23	25	15	25	23
Unpaid Principal Balance	15	23	25	15	25	23
Average Recorded Investment	16		26	19	28	27
Interest Income Recognized			21	1	37
Interest Income Recognized						1
With no related allowance recorded, Recorded Investment	15	23	25	15	25	23
With no related allowance recorded, Unpaid Principal Balance	15	$ 23	25	15	25	23
With no related allowance recorded, Average Recorded Investment	$ 16		26	19	21	$ 16
With no related allowance recorded, Interest Income Recognized			$ 21	$ 1	$ 37

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Impaired financing receivable with no related allowance interest income accrual method immaterial amount.

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Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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Amount of allowance for credit losses related to recorded investment.

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Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

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