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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	$ 3,041		$ 3,041		$ 4,523
With no related allowance, Borrowers' Unpaid Principal Balance	3,112		3,112		5,251
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	4,553		4,553		4,584
With an allowance recorded, Borrowers' Unpaid Principal Balance	4,553		4,553		4,694
With an allowance recorded, Related Allowance	1,053		1,053		1,094
Total Impaired Loans, Recorded Investment	7,594		7,594		9,107
Total Impaired Loans, Borrowers' Unpaid Principal Balance	7,665		7,665		9,945
Total Impaired Loans, Related Allowance	1,053		1,053		1,094
With no related allowance, Average Recorded Investment	2,994	$ 3,552	2,829	$ 3,659
With no related allowance, Interest Income Recognized	33	18	96	49
With an allowance recorded, Average Recorded Investment	4,428	6,096	4,082	6,078
With an allowance recorded, Interest Income Recognized	66	53	183	172
Total, Average Recorded Investment	7,422	9,648	6,911	9,737
Total, Interest Income Recognized	99	71	279	221
Construction, land and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	92		92		900
With no related allowance, Borrowers' Unpaid Principal Balance	163		163		1,378
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	523		523		550
With an allowance recorded, Borrowers' Unpaid Principal Balance	523		523		621
With an allowance recorded, Related Allowance	290		290		137
Total Impaired Loans, Recorded Investment	615		615		1,450
Total Impaired Loans, Borrowers' Unpaid Principal Balance	686		686		1,999
Total Impaired Loans, Related Allowance	290		290		137
With no related allowance, Average Recorded Investment	93	367	147	385
With no related allowance, Interest Income Recognized			1
With an allowance recorded, Average Recorded Investment	524	1,315	512	1,323
With an allowance recorded, Interest Income Recognized	8	15	24	44
Total, Average Recorded Investment	617	1,682	659	1,708
Total, Interest Income Recognized	8	15	25	44
Commercial mortgages (non-owner occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	443		443
With an allowance recorded, Borrowers' Unpaid Principal Balance	443		443
With an allowance recorded, Related Allowance	18		18
Total Impaired Loans, Recorded Investment	443		443
Total Impaired Loans, Borrowers' Unpaid Principal Balance	443		443
Total Impaired Loans, Related Allowance	18		18
With no related allowance, Average Recorded Investment		248		248
With no related allowance, Interest Income Recognized		4		11
With an allowance recorded, Average Recorded Investment	445		223
With an allowance recorded, Interest Income Recognized	11		11
Total, Average Recorded Investment	445	248	223	248
Total, Interest Income Recognized	11	4	11	11
Commercial mortgages (owner occupied)
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,026		1,026		1,721
With no related allowance, Borrowers' Unpaid Principal Balance	1,026		1,026		1,971
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	931		931		547
With an allowance recorded, Borrowers' Unpaid Principal Balance	931		931		586
With an allowance recorded, Related Allowance	156		156		195
Total Impaired Loans, Recorded Investment	1,957		1,957		2,268
Total Impaired Loans, Borrowers' Unpaid Principal Balance	1,957		1,957		2,557
Total Impaired Loans, Related Allowance	156		156		195
With no related allowance, Average Recorded Investment	1,013	1,030	985	1,102
With no related allowance, Interest Income Recognized	8	5	29	16
With an allowance recorded, Average Recorded Investment	935	1,372	945	1,381
With an allowance recorded, Interest Income Recognized	13	4	39	19
Total, Average Recorded Investment	1,948	2,402	1,930	2,483
Total, Interest Income Recognized	21	9	68	35
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	1,507		1,507		1,488
With no related allowance, Borrowers' Unpaid Principal Balance	1,507		1,507		1,488
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	2,395		2,395		1,914
With an allowance recorded, Borrowers' Unpaid Principal Balance	2,395		2,395		1,914
With an allowance recorded, Related Allowance	374		374		367
Total Impaired Loans, Recorded Investment	3,902		3,902		3,402
Total Impaired Loans, Borrowers' Unpaid Principal Balance	3,902		3,902		3,402
Total Impaired Loans, Related Allowance	374		374		367
With no related allowance, Average Recorded Investment	1,472	1,425	1,282	1,439
With no related allowance, Interest Income Recognized	21	4	62	15
With an allowance recorded, Average Recorded Investment	2,394	1,928	2,277	1,937
With an allowance recorded, Interest Income Recognized	32	23	94	71
Total, Average Recorded Investment	3,866	3,353	3,559	3,376
Total, Interest Income Recognized	53	27	156	86
Residential revolving and junior mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Recorded Investment	416		416		414
With no related allowance, Borrowers' Unpaid Principal Balance	416		416		414
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	130		130		1,340
With an allowance recorded, Borrowers' Unpaid Principal Balance	130		130		1,340
With an allowance recorded, Related Allowance	84		84		162
Total Impaired Loans, Recorded Investment	546		546		1,754
Total Impaired Loans, Borrowers' Unpaid Principal Balance	546		546		1,754
Total Impaired Loans, Related Allowance	84		84		162
With no related allowance, Average Recorded Investment	416	482	415	485
With no related allowance, Interest Income Recognized	1	5	4	7
With an allowance recorded, Average Recorded Investment	130	1,389	125	1,345
With an allowance recorded, Interest Income Recognized	2	11	7	38
Total, Average Recorded Investment	546	1,871	540	1,830
Total, Interest Income Recognized	3	16	11	45
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					92
With an allowance recorded, Borrowers' Unpaid Principal Balance					92
With an allowance recorded, Related Allowance					92
Total Impaired Loans, Recorded Investment					92
Total Impaired Loans, Borrowers' Unpaid Principal Balance					92
Total Impaired Loans, Related Allowance					92
With an allowance recorded, Average Recorded Investment		92		92
Total, Average Recorded Investment		$ 92		$ 92
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
With no related allowance, Related Allowance	0		0		0
With an allowance recorded, Recorded Investment	131		131		141
With an allowance recorded, Borrowers' Unpaid Principal Balance	131		131		141
With an allowance recorded, Related Allowance	131		131		141
Total Impaired Loans, Recorded Investment	131		131		141
Total Impaired Loans, Borrowers' Unpaid Principal Balance	131		131		141
Total Impaired Loans, Related Allowance	131		131		$ 141
With no related allowance, Interest Income Recognized	3
With an allowance recorded, Interest Income Recognized			8
Total, Interest Income Recognized	$ 3		$ 8