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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 2,142		$ 2,142		$ 1,749
With no related allowance, Borrowers' Unpaid Principal Balance	2,213		2,213		1,820
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	5,335		5,335		5,857
With an allowance recorded, Borrowers' Unpaid Principal Balance	5,335		5,335		5,857
With an allowance recorded, Related Allowance	1,106		1,106		1,157
Total Impaired Loans, Recorded Investment	7,477		7,477		7,606
Total Impaired Loans, Borrowers' Unpaid Principal Balance	7,548		7,548		7,677
With no related allowance, Average Recorded Investment	2,124	$ 3,397	2,122	$ 3,431
With no related allowance, Interest Income Recognized	29	48	58	77
With an allowance recorded, Average Recorded Investment	5,397	4,087	5,427	3,752
With an allowance recorded, Interest Income Recognized	67	54	146	108
Total, Average Recorded Investment	7,521	7,484	7,549	7,183
Total, Interest Income Recognized	96	102	204	185
Construction, land and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	328		328		335
With no related allowance, Borrowers' Unpaid Principal Balance	399		399		406
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	176		176		275
With an allowance recorded, Borrowers' Unpaid Principal Balance	176		176		275
With an allowance recorded, Related Allowance	39		39		132
Total Impaired Loans, Recorded Investment	504		504		610
Total Impaired Loans, Borrowers' Unpaid Principal Balance	575		575		681
With no related allowance, Average Recorded Investment	330	84	332	165
With no related allowance, Interest Income Recognized	4		9	1
With an allowance recorded, Average Recorded Investment	225	571	241	553
With an allowance recorded, Interest Income Recognized	10	8	15	17
Total, Average Recorded Investment	555	655	573	718
Total, Interest Income Recognized	14	8	24	18
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	434		434		443
With an allowance recorded, Borrowers' Unpaid Principal Balance	434		434		443
With an allowance recorded, Related Allowance	34		34		18
Total Impaired Loans, Recorded Investment	434		434		829
Total Impaired Loans, Borrowers' Unpaid Principal Balance	434		434		829
With an allowance recorded, Average Recorded Investment	435	224	437	149
With an allowance recorded, Interest Income Recognized	4		27
Total, Average Recorded Investment	435	224	437	149
Total, Interest Income Recognized	4		27
Commercial Mortgages (Owner Occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	429		429
With no related allowance, Borrowers' Unpaid Principal Balance	429		429
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	1,058		1,058		1,069
With an allowance recorded, Borrowers' Unpaid Principal Balance	1,058		1,058		1,069
With an allowance recorded, Related Allowance	54		54		57
Total Impaired Loans, Recorded Investment	1,487		1,487		1,069
Total Impaired Loans, Borrowers' Unpaid Principal Balance	1,487		1,487		1,069
With no related allowance, Average Recorded Investment	405	1,385	399	1,380
With no related allowance, Interest Income Recognized	7	20	13	32
With an allowance recorded, Average Recorded Investment	1,061	538	1,063	541
With an allowance recorded, Interest Income Recognized	14	8	28	15
Total, Average Recorded Investment	1,466	1,923	1,462	1,921
Total, Interest Income Recognized	21	28	41	47
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,385		1,385
With no related allowance, Borrowers' Unpaid Principal Balance	1,385		1,385
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	3,048		3,048		3,447
With an allowance recorded, Borrowers' Unpaid Principal Balance	3,048		3,048		3,447
With an allowance recorded, Related Allowance	644		644		565
Total Impaired Loans, Recorded Investment	4,433		4,433		3,447
Total Impaired Loans, Borrowers' Unpaid Principal Balance	4,433		4,433		3,447
With no related allowance, Average Recorded Investment	1,389	1,513	1,391	1,471
With no related allowance, Interest Income Recognized	18	24	36	41
With an allowance recorded, Average Recorded Investment	3,055	2,623	3,065	2,386
With an allowance recorded, Interest Income Recognized	34	36	65	67
Total, Average Recorded Investment	4,444	4,136	4,456	3,857
Total, Interest Income Recognized	52	60	101	108
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	500		500		502
With an allowance recorded, Borrowers' Unpaid Principal Balance	500		500		502
With an allowance recorded, Related Allowance	216		216		264
Total Impaired Loans, Recorded Investment	500		500		1,530
Total Impaired Loans, Borrowers' Unpaid Principal Balance	500		500		1,530
With no related allowance, Average Recorded Investment		415		415
With no related allowance, Interest Income Recognized		1		3
With an allowance recorded, Average Recorded Investment	501	131	501	123
With an allowance recorded, Interest Income Recognized	3	2	6	4
Total, Average Recorded Investment	501	546	501	538
Total, Interest Income Recognized	3	3	6	7
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	119		119		121
With an allowance recorded, Borrowers' Unpaid Principal Balance	119		119		121
With an allowance recorded, Related Allowance	119		119		121
Total Impaired Loans, Recorded Investment	119		119		121
Total Impaired Loans, Borrowers' Unpaid Principal Balance	119		119		$ 121
With no related allowance, Interest Income Recognized		3
With an allowance recorded, Average Recorded Investment	120		120
With an allowance recorded, Interest Income Recognized	2		5	5
Total, Average Recorded Investment	120		120
Total, Interest Income Recognized	$ 2	$ 3	$ 5	$ 5