Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) Contract	Dec. 31, 2019 USD ($) Contract	Dec. 31, 2018 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	41	38	58
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 14,874	$ 8,302	$ 11,281
Financing Receivable, Troubled Debt Restructuring	13,969	1,928	8,829
Financing Receivable Troubled Debt Restructuring Specific Reserve	636	0	0
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	14,098	7,525	10,635
Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	776	760	330
Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0	$ 17	$ 316
Commercial Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	4	2	3
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 2,805	$ 156	$ 8,324
Financing Receivable, Troubled Debt Restructuring	2,196	154	6,104
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 636	0	0
Commercial Loan [Member] | Time and demand loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	4
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 2,805
Financing Receivable, Troubled Debt Restructuring	2,196
Financing Receivable Troubled Debt Restructuring Specific Reserve	636
Commercial Loan [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	2,176	156	8,250
Commercial Loan [Member] | Contractual Payment Modification [Member] | Time and demand loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	2,176
Commercial Loan [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	629	0	0
Commercial Loan [Member] | Contractual Interest Rate Reduction [Member] | Time and demand loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	629
Commercial Loan [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	$ 0	$ 74
Commercial Loan [Member] | Extended Maturity [Member] | Time and demand loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0
Residential Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	18	20	37
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 950	$ 1,186	$ 1,799
Financing Receivable, Troubled Debt Restructuring	791	1,059	1,638
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 0	0	0
Residential Real Estate [Member] | Senior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	9
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 546
Financing Receivable, Troubled Debt Restructuring	411
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 0
Residential Real Estate [Member] | Junior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	9
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 404
Financing Receivable, Troubled Debt Restructuring	380
Financing Receivable Troubled Debt Restructuring Specific Reserve	0
Residential Real Estate [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	917	965	1,316
Residential Real Estate [Member] | Contractual Payment Modification [Member] | Senior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	513
Residential Real Estate [Member] | Contractual Payment Modification [Member] | Junior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	404
Residential Real Estate [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	33	204	241
Residential Real Estate [Member] | Contractual Interest Rate Reduction [Member] | Senior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	33
Residential Real Estate [Member] | Contractual Interest Rate Reduction [Member] | Junior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0
Residential Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	$ 17	$ 242
Residential Real Estate [Member] | Extended Maturity [Member] | Senior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0
Residential Real Estate [Member] | Extended Maturity [Member] | Junior Lien
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	5	5	3
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 10,857	$ 6,817	$ 1,016
Financing Receivable, Troubled Debt Restructuring	10,758	594	975
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 0	0	0
Commercial Real Estate [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	4
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 10,289
Financing Receivable, Troubled Debt Restructuring	10,263
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 0
Commercial Real Estate [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	1
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 568
Financing Receivable, Troubled Debt Restructuring	495
Financing Receivable Troubled Debt Restructuring Specific Reserve	0
Commercial Real Estate [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	10,857	6,261	1,016
Commercial Real Estate [Member] | Contractual Payment Modification [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	10,289
Commercial Real Estate [Member] | Contractual Payment Modification [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	568
Commercial Real Estate [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	556	0
Commercial Real Estate [Member] | Contractual Interest Rate Reduction [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0
Commercial Real Estate [Member] | Contractual Interest Rate Reduction [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0
Commercial Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	$ 0	$ 0
Commercial Real Estate [Member] | Extended Maturity [Member] | Nonowner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0
Commercial Real Estate [Member] | Extended Maturity [Member] | Owner Occupied Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0
Loans To Individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	14	11	15
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 262	$ 143	$ 142
Financing Receivable, Troubled Debt Restructuring	224	121	112
Financing Receivable Troubled Debt Restructuring Specific Reserve	$ 0	0	0
Loans To Individuals [Member] | Automobiles
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	14
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 262
Financing Receivable, Troubled Debt Restructuring	224
Financing Receivable Troubled Debt Restructuring Specific Reserve	0
Loans To Individuals [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	148	143	53
Loans To Individuals [Member] | Contractual Payment Modification [Member] | Automobiles
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	148
Loans To Individuals [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	114	0	89
Loans To Individuals [Member] | Contractual Interest Rate Reduction [Member] | Automobiles
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	114
Loans To Individuals [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	$ 0	$ 0
Loans To Individuals [Member] | Extended Maturity [Member] | Automobiles
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0