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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 1,776		$ 1,776		$ 1,749
With no related allowance, Borrowers' Unpaid Principal Balance	1,847		1,847		1,820
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	7,331		7,331		5,857
With an allowance recorded, Borrowers' Unpaid Principal Balance	7,331		7,331		5,857
With an allowance recorded, Related Allowance	1,007		1,007		1,157
Total Impaired Loans, Recorded Investment	9,107		9,107		7,606
Total Impaired Loans, Borrowers' Unpaid Principal Balance	9,178		9,178		7,677
With no related allowance, Average Recorded Investment	1,792	$ 2,994	1,848	$ 2,829
With no related allowance, Interest Income Recognized	21	33	67	96
With an allowance recorded, Average Recorded Investment	6,293	4,428	6,090	4,082
With an allowance recorded, Interest Income Recognized	88	66	226	183
Total, Average Recorded Investment	8,085	7,422	7,938	6,911
Total, Interest Income Recognized	109	99	293	279
Construction, land and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	324		324		335
With no related allowance, Borrowers' Unpaid Principal Balance	395		395		406
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	173		173		275
With an allowance recorded, Borrowers' Unpaid Principal Balance	173		173		275
With an allowance recorded, Related Allowance	38		38		132
Total Impaired Loans, Recorded Investment	497		497		610
Total Impaired Loans, Borrowers' Unpaid Principal Balance	568		568		681
With no related allowance, Average Recorded Investment	326	93	330	147
With no related allowance, Interest Income Recognized	1		4	1
With an allowance recorded, Average Recorded Investment	175	524	224	512
With an allowance recorded, Interest Income Recognized	3	8	18	24
Total, Average Recorded Investment	501	617	554	659
Total, Interest Income Recognized	4	8	22	25
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		0
With an allowance recorded, Recorded Investment	433		433		443
With an allowance recorded, Borrowers' Unpaid Principal Balance	433		433		443
With an allowance recorded, Related Allowance	33		33		18
Total Impaired Loans, Recorded Investment	433		433		829
Total Impaired Loans, Borrowers' Unpaid Principal Balance	433		433		829
With an allowance recorded, Average Recorded Investment	434	445	436	223
With an allowance recorded, Interest Income Recognized	3	11	30	11
Total, Average Recorded Investment	434	445	436	223
Total, Interest Income Recognized	3	11	30	11
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	425		425
With no related allowance, Borrowers' Unpaid Principal Balance	425		425
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	1,052		1,052		1,069
With an allowance recorded, Borrowers' Unpaid Principal Balance	1,052		1,052		1,069
With an allowance recorded, Related Allowance	53		53		57
Total Impaired Loans, Recorded Investment	1,477		1,477		1,069
Total Impaired Loans, Borrowers' Unpaid Principal Balance	1,477		1,477		1,069
With no related allowance, Average Recorded Investment	427	1,013	405	985
With no related allowance, Interest Income Recognized	6	8	20	29
With an allowance recorded, Average Recorded Investment	1,055	935	1,061	945
With an allowance recorded, Interest Income Recognized	14	13	42	39
Total, Average Recorded Investment	1,482	1,948	1,466	1,930
Total, Interest Income Recognized	20	21	62	68
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,027		1,027
With no related allowance, Borrowers' Unpaid Principal Balance	1,027		1,027
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	2,785		2,785		3,447
With an allowance recorded, Borrowers' Unpaid Principal Balance	2,785		2,785		3,447
With an allowance recorded, Related Allowance	543		543		565
Total Impaired Loans, Recorded Investment	3,812		3,812		3,447
Total Impaired Loans, Borrowers' Unpaid Principal Balance	3,812		3,812		3,447
With no related allowance, Average Recorded Investment	1,039	1,472	1,113	1,282
With no related allowance, Interest Income Recognized	14	21	43	62
With an allowance recorded, Average Recorded Investment	3,039	2,394	3,181	2,277
With an allowance recorded, Interest Income Recognized	34	32	93	94
Total, Average Recorded Investment	4,078	3,866	4,294	3,559
Total, Interest Income Recognized	48	53	136	156
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		0
With an allowance recorded, Recorded Investment	83		83		502
With an allowance recorded, Borrowers' Unpaid Principal Balance	83		83		502
With an allowance recorded, Related Allowance	83		83		264
Total Impaired Loans, Recorded Investment	83		83		1,530
Total Impaired Loans, Borrowers' Unpaid Principal Balance	83		83		1,530
With no related allowance, Average Recorded Investment		416		415
With no related allowance, Interest Income Recognized		1		4
With an allowance recorded, Average Recorded Investment	128	130	397	125
With an allowance recorded, Interest Income Recognized	2	2	7	7
Total, Average Recorded Investment	128	546	397	540
Total, Interest Income Recognized	2	3	7	11
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	2,689		2,689
With an allowance recorded, Borrowers' Unpaid Principal Balance	2,689		2,689
With an allowance recorded, Related Allowance	141		141
Total Impaired Loans, Recorded Investment	2,689		2,689
Total Impaired Loans, Borrowers' Unpaid Principal Balance	2,689		2,689
With an allowance recorded, Average Recorded Investment	1,344		672
With an allowance recorded, Interest Income Recognized	30		30
Total, Average Recorded Investment	1,344		672
Total, Interest Income Recognized	30		30
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	116		116		121
With an allowance recorded, Borrowers' Unpaid Principal Balance	116		116		121
With an allowance recorded, Related Allowance	116		116		121
Total Impaired Loans, Recorded Investment	116		116		121
Total Impaired Loans, Borrowers' Unpaid Principal Balance	116		116		$ 121
With no related allowance, Interest Income Recognized		3
With an allowance recorded, Average Recorded Investment	118		119
With an allowance recorded, Interest Income Recognized	2		6	8
Total, Average Recorded Investment	118		119
Total, Interest Income Recognized	$ 2	$ 3	$ 6	$ 8