Allowance for Loan Losses - Company's Recorded Investment and Borrowers' Unpaid Principal Balances for Impaired Loans, Excluding PCI Loans, with Associated ALL Amount (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 946	$ 1,749
With no related allowance, Borrowers' Unpaid Principal Balance	946	1,820
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	5,123	5,857
With an allowance recorded, Borrowers' Unpaid Principal Balance	5,123	5,857
With an allowance recorded, Related Allowance	1,039	1,157
Total Impaired Loans, Recorded Investment	6,069	7,606
Total Impaired Loans, Borrowers' Unpaid Principal Balance	6,069	7,677
With no related allowance, Average Recorded Investment	940	1,536
With no related allowance, Interest Income Recognized	55	81
With an allowance recorded, Average Recorded Investment	5,087	4,914
With an allowance recorded, Interest Income Recognized	250	244
Total, Average Recorded Investment	6,027	6,450
Total, Interest Income Recognized	305	325
Construction, land and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	17	335
With no related allowance, Borrowers' Unpaid Principal Balance	17	406
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	171	275
With an allowance recorded, Borrowers' Unpaid Principal Balance	171	275
With an allowance recorded, Related Allowance	44	132
Total Impaired Loans, Recorded Investment	188	610
Total Impaired Loans, Borrowers' Unpaid Principal Balance	188	681
With no related allowance, Average Recorded Investment	18	382
With no related allowance, Interest Income Recognized	1	13
With an allowance recorded, Average Recorded Investment	214	267
With an allowance recorded, Interest Income Recognized	21	18
Total, Average Recorded Investment	232	649
Total, Interest Income Recognized	22	31
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	510	0
With no related allowance, Borrowers' Unpaid Principal Balance	510	0
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	2,857	3,447
With an allowance recorded, Borrowers' Unpaid Principal Balance	2,857	3,447
With an allowance recorded, Related Allowance	676	565
Total Impaired Loans, Recorded Investment	3,367	3,447
Total Impaired Loans, Borrowers' Unpaid Principal Balance	3,367	3,447
With no related allowance, Average Recorded Investment	514	760
With no related allowance, Interest Income Recognized	27	0
With an allowance recorded, Average Recorded Investment	2,586	3,037
With an allowance recorded, Interest Income Recognized	100	150
Total, Average Recorded Investment	3,100	3,797
Total, Interest Income Recognized	127	194
Residential revolving and junior mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		1,028
With no related allowance, Borrowers' Unpaid Principal Balance		1,028
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	47	502
With an allowance recorded, Borrowers' Unpaid Principal Balance	47	502
With an allowance recorded, Related Allowance	47	264
Total Impaired Loans, Recorded Investment	47	1,530
Total Impaired Loans, Borrowers' Unpaid Principal Balance	47	1,530
With no related allowance, Interest Income Recognized		44
With an allowance recorded, Average Recorded Investment	47	149
With an allowance recorded, Interest Income Recognized	7	6
Total, Average Recorded Investment	47	149
Total, Interest Income Recognized	7	6
Commercial Mortgages (Non-Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment		386
With no related allowance, Borrowers' Unpaid Principal Balance		386
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	433	443
With an allowance recorded, Borrowers' Unpaid Principal Balance	433	443
With an allowance recorded, Related Allowance	58	18
Total Impaired Loans, Recorded Investment	433	829
Total Impaired Loans, Borrowers' Unpaid Principal Balance	433	829
With no related allowance, Average Recorded Investment	0	0
With no related allowance, Interest Income Recognized	0	0
With an allowance recorded, Average Recorded Investment	436	267
With an allowance recorded, Interest Income Recognized	32	12
Total, Average Recorded Investment	436	267
Total, Interest Income Recognized	32	12
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	419
With no related allowance, Borrowers' Unpaid Principal Balance	419
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	1,048	1,069
With an allowance recorded, Borrowers' Unpaid Principal Balance	1,048	1,069
With an allowance recorded, Related Allowance	53	57
Total Impaired Loans, Recorded Investment	1,467	1,069
Total Impaired Loans, Borrowers' Unpaid Principal Balance	1,467	1,069
With no related allowance, Average Recorded Investment	408	394
With no related allowance, Interest Income Recognized	27	24
With an allowance recorded, Average Recorded Investment	1,058	1,061
With an allowance recorded, Interest Income Recognized	56	47
Total, Average Recorded Investment	1,466	1,455
Total, Interest Income Recognized	83	71
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	0	0
With no related allowance, Borrowers' Unpaid Principal Balance	0	0
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	455
With an allowance recorded, Borrowers' Unpaid Principal Balance	455
With an allowance recorded, Related Allowance	49
Total Impaired Loans, Recorded Investment	455
Total Impaired Loans, Borrowers' Unpaid Principal Balance	455
With no related allowance, Average Recorded Investment	0	0
With no related allowance, Interest Income Recognized	0	0
With an allowance recorded, Average Recorded Investment	628
With an allowance recorded, Interest Income Recognized	30	0
Total, Average Recorded Investment	628
Total, Interest Income Recognized	30	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	0	0
With no related allowance, Borrowers' Unpaid Principal Balance	0	0
With no related allowance, Related Allowance	0	0
With an allowance recorded, Recorded Investment	112	121
With an allowance recorded, Borrowers' Unpaid Principal Balance	112	121
With an allowance recorded, Related Allowance	112	121
Total Impaired Loans, Recorded Investment	112	121
Total Impaired Loans, Borrowers' Unpaid Principal Balance	112	121
With no related allowance, Average Recorded Investment	0	0
With no related allowance, Interest Income Recognized	0	0
With an allowance recorded, Average Recorded Investment	118	133
With an allowance recorded, Interest Income Recognized	4	11
Total, Average Recorded Investment	118	133
Total, Interest Income Recognized	$ 4	$ 11