Allowance for Loan Losses - Summary of Recorded Investment and Borrowers' Unpaid Principal Balances for Impaired Loans, Excluding PCI Loans, with Associated ALL Amount (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 5,372		$ 5,372		$ 946
With no related allowance, Borrowers' Unpaid Principal Balance	5,372		5,372		946
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	12,736		12,736		5,123
With an allowance recorded, Borrowers' Unpaid Principal Balance	12,736		12,736		5,123
With an allowance recorded, Related Allowance	2,887		2,887		1,039
Total Impaired Loans, Recorded Investment	18,108		18,108		6,069
Total Impaired Loans, Borrowers' Unpaid Principal Balance	18,108		18,108		6,069
With no related allowance, Average Recorded Investment	5,379	$ 1,792	3,044	$ 1,848
With no related allowance, Interest Income Recognized	51	21	108	67
With an allowance recorded, Average Recorded Investment	10,369	6,293	7,495	6,090
With an allowance recorded, Interest Income Recognized	119	88	231	226
Total, Average Recorded Investment	15,748	8,085	10,539	7,938
Total, Interest Income Recognized	170	109	339	293
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	712		712		510
With no related allowance, Borrowers' Unpaid Principal Balance	712		712		510
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	3,585		3,585		2,857
With an allowance recorded, Borrowers' Unpaid Principal Balance	3,585		3,585		2,857
With an allowance recorded, Related Allowance	763		763		676
Total Impaired Loans, Recorded Investment	4,297		4,297		3,367
Total Impaired Loans, Borrowers' Unpaid Principal Balance	4,297		4,297		3,367
With no related allowance, Average Recorded Investment	714	1,039	519	1,113
With no related allowance, Interest Income Recognized	8	14	20	43
With an allowance recorded, Average Recorded Investment	3,595	3,039	3,262	3,181
With an allowance recorded, Interest Income Recognized	19	34	55	93
Total, Average Recorded Investment	4,309	4,078	3,781	4,294
Total, Interest Income Recognized	27	48	75	136
Commercial mortgages (non-owner occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	3,270		3,270		0
With no related allowance, Borrowers' Unpaid Principal Balance	3,270		3,270		0
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	2,838		2,838		433
With an allowance recorded, Borrowers' Unpaid Principal Balance	2,838		2,838		433
With an allowance recorded, Related Allowance	651		651		58
Total Impaired Loans, Recorded Investment	6,108		6,108		433
Total Impaired Loans, Borrowers' Unpaid Principal Balance	6,108		6,108		433
With no related allowance, Average Recorded Investment	3,263	0	1,632	0
With no related allowance, Interest Income Recognized	23	0	43	0
With an allowance recorded, Average Recorded Investment	1,635	434	1,034	436
With an allowance recorded, Interest Income Recognized	22	3	32	30
Total, Average Recorded Investment	4,898	434	2,666	436
Total, Interest Income Recognized	45	3	75	30
Construction, land and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	15		15		17
With no related allowance, Borrowers' Unpaid Principal Balance	15		15		17
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	766		766		171
With an allowance recorded, Borrowers' Unpaid Principal Balance	766		766		171
With an allowance recorded, Related Allowance	546		546		44
Total Impaired Loans, Recorded Investment	781		781		188
Total Impaired Loans, Borrowers' Unpaid Principal Balance	781		781		188
With no related allowance, Average Recorded Investment	15	326	16	330
With no related allowance, Interest Income Recognized	0	1	1	4
With an allowance recorded, Average Recorded Investment	768	175	468	224
With an allowance recorded, Interest Income Recognized	5	3	11	18
Total, Average Recorded Investment	783	501	484	554
Total, Interest Income Recognized	5	4	12	22
Commercial mortgages (owner occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	361		361		419
With no related allowance, Borrowers' Unpaid Principal Balance	361		361		419
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	657		657		1,048
With an allowance recorded, Borrowers' Unpaid Principal Balance	657		657		1,048
With an allowance recorded, Related Allowance	31		31		53
Total Impaired Loans, Recorded Investment	1,018		1,018		1,467
Total Impaired Loans, Borrowers' Unpaid Principal Balance	1,018		1,018		1,467
With no related allowance, Average Recorded Investment	363	427	365	405
With no related allowance, Interest Income Recognized	7	6	19	20
With an allowance recorded, Average Recorded Investment	656	1,055	658	1,061
With an allowance recorded, Interest Income Recognized	8	14	25	42
Total, Average Recorded Investment	1,019	1,482	1,023	1,466
Total, Interest Income Recognized	15	20	44	62
Residential revolving and junior mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	0		0		0
With no related allowance, Borrowers' Unpaid Principal Balance	0		0		0
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	45		45		47
With an allowance recorded, Borrowers' Unpaid Principal Balance	45		45		47
With an allowance recorded, Related Allowance	45		45		47
Total Impaired Loans, Recorded Investment	45		45		47
Total Impaired Loans, Borrowers' Unpaid Principal Balance	45		45		47
With no related allowance, Average Recorded Investment	0	0	0	0
With no related allowance, Interest Income Recognized	0	0	0	0
With an allowance recorded, Average Recorded Investment	46	128	46	397
With an allowance recorded, Interest Income Recognized	1	2	3	7
Total, Average Recorded Investment	46	128	46	397
Total, Interest Income Recognized	1	2	3	7
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,014		1,014		0
With no related allowance, Borrowers' Unpaid Principal Balance	1,014		1,014		0
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	4,740		4,740		455
With an allowance recorded, Borrowers' Unpaid Principal Balance	4,740		4,740		455
With an allowance recorded, Related Allowance	746		746		49
Total Impaired Loans, Recorded Investment	5,754		5,754		455
Total Impaired Loans, Borrowers' Unpaid Principal Balance	5,754		5,754		455
With no related allowance, Average Recorded Investment	1,024	0	512	0
With no related allowance, Interest Income Recognized	13	0	25	0
With an allowance recorded, Average Recorded Investment	3,561	1,344	1,918	672
With an allowance recorded, Interest Income Recognized	64	30	105	30
Total, Average Recorded Investment	4,585	1,344	2,430	672
Total, Interest Income Recognized	77	30	130	30
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	0		0		0
With no related allowance, Borrowers' Unpaid Principal Balance	0		0		0
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	105		105		112
With an allowance recorded, Borrowers' Unpaid Principal Balance	105		105		112
With an allowance recorded, Related Allowance	105		105		112
Total Impaired Loans, Recorded Investment	105		105		112
Total Impaired Loans, Borrowers' Unpaid Principal Balance	105		105		$ 112
With no related allowance, Average Recorded Investment	0	0	0	0
With no related allowance, Interest Income Recognized	0	0	0	0
With an allowance recorded, Average Recorded Investment	108	118	109	119
With an allowance recorded, Interest Income Recognized	0	2	0	6
Total, Average Recorded Investment	108	118	109	119
Total, Interest Income Recognized	$ 0	$ 2	$ 0	$ 6