Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) Contract	Dec. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) Contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	38	58	41
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 8,302	$ 11,281	$ 11,054
Financing Receivable, Troubled Debt Restructuring	1,928	8,829	8,274
Financing Receivable Troubled Debt Restructuring Specific Reserve	0	0	596
Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	7,525	10,635	1,878
Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	760	330	2,095
Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 17	$ 316	$ 7,081
Commercial Loan [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	2	3	6
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 156	$ 8,324	$ 9,561
Financing Receivable, Troubled Debt Restructuring	154	6,104	6,946
Financing Receivable Troubled Debt Restructuring Specific Reserve	0	0	566
Commercial Loan [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	156	8,250	987
Commercial Loan [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	0	1,806
Commercial Loan [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0	$ 74	$ 6,768
Residential Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	20	37	20
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 1,186	$ 1,799	$ 968
Financing Receivable, Troubled Debt Restructuring	1,059	1,638	851
Financing Receivable Troubled Debt Restructuring Specific Reserve	0	0	1
Residential Real Estate [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	965	1,316	573
Residential Real Estate [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	204	241	261
Residential Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 17	$ 242	$ 134
Commercial Real Estate [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	5	3	5
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 6,817	$ 1,016	$ 448
Financing Receivable, Troubled Debt Restructuring	594	975	412
Financing Receivable Troubled Debt Restructuring Specific Reserve	0	0	29
Commercial Real Estate [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	6,261	1,016	269
Commercial Real Estate [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	556	0	0
Commercial Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0	$ 0	$ 179
Loans To Individuals [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Modifications, Number of Contracts | Contract	11	15	10
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 143	$ 142	$ 77
Financing Receivable, Troubled Debt Restructuring	121	112	65
Financing Receivable Troubled Debt Restructuring Specific Reserve	0	0	0
Loans To Individuals [Member] | Contractual Payment Modification [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	143	53	49
Loans To Individuals [Member] | Contractual Interest Rate Reduction [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	0	89	28
Loans To Individuals [Member] | Extended Maturity [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Troubled Debt Restructuring, Premodification	$ 0	$ 0	$ 0