Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Originated [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 25,607	$ 29,012	$ 26,175
Recorded investment with no related allowance	18,311	22,425	18,078
Recorded investment with related allowance	5,397	5,162	7,188
Total recorded investment	23,708	27,587	25,266
Related Allowance	2,234	1,563	2,232
Average recorded investment	23,292	25,787	24,626
Interest income recognized	409	712	1,052
Originated [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	16,738	22,132	18,209
Recorded investment with no related allowance	12,870	18,816	15,159
Recorded investment with related allowance	2,681	2,794	2,691
Total recorded investment	15,551	21,610	17,850
Related Allowance	325	240	260
Average recorded investment	15,432	19,232	17,352
Interest income recognized	275	604	788
Originated [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,001	3,474	3,260
Recorded investment with no related allowance	3,306	2,581	2,204
Recorded investment with related allowance	219	320	602
Total recorded investment	3,525	2,901	2,806
Related Allowance	52	53	124
Average recorded investment	3,166	2,963	2,915
Interest income recognized	56	32	71
Originated [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,868	3,262	4,566
Recorded investment with no related allowance	2,135	884	575
Recorded investment with related allowance	2,497	2,048	3,895
Total recorded investment	4,632	2,932	4,470
Related Allowance	1,857	1,270	1,848
Average recorded investment	4,626	3,514	4,283
Interest income recognized	78	69	184
Originated [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		144	140
Recorded investment with no related allowance		144	140
Total recorded investment		144	140
Average recorded investment	68	78	76
Interest income recognized		7	9
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,185	3,489	4,023
Recorded investment with no related allowance	3,251	1,777	2,058
Recorded investment with related allowance	311	1,644	1,881
Total recorded investment	3,562	3,421	3,939
Related Allowance	57	167	230
Average recorded investment	3,883	3,242	3,501
Interest income recognized	67	90	143
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,554	2,091	1,627
Recorded investment with no related allowance	1,960	1,470	1,107
Recorded investment with related allowance	157	199	198
Total recorded investment	2,117	1,669	1,305
Related Allowance	30	13	10
Average recorded investment	1,753	1,376	1,193
Interest income recognized	24	32	24
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3	59	52
Recorded investment with no related allowance		3	4
Interest loss recognized		(25)
Recorded investment with related allowance	3	11	3
Total recorded investment	3	14	7
Related Allowance	3	7	3
Average recorded investment	3	23	20
Originated [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	12,553	18,643	14,186
Recorded investment with no related allowance	9,619	17,039	13,101
Recorded investment with related allowance	2,370	1,150	810
Total recorded investment	11,989	18,189	13,911
Related Allowance	268	73	30
Average recorded investment	11,549	15,990	13,851
Interest income recognized	208	514	645
Originated [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,444	1,324	1,581
Recorded investment with no related allowance	1,346	1,108	1,093
Recorded investment with related allowance	59	110	401
Total recorded investment	1,405	1,218	1,494
Related Allowance	19	33	111
Average recorded investment	1,410	1,564	1,702
Interest income recognized	32	25	47
Originated [Member] | Residential [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		144	140
Recorded investment with no related allowance		144	140
Total recorded investment		144	140
Average recorded investment	68	78	76
Interest income recognized		7	9
PNCI [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,945	4,967	3,048
Recorded investment with no related allowance	3,149	3,399	1,994
Recorded investment with related allowance	637	1,233	974
Total recorded investment	3,786	4,632	2,968
Related Allowance	394	577	467
Average recorded investment	3,355	4,397	3,563
Interest income recognized	87	65	89
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,557	3,503	1,404
Recorded investment with no related allowance	2,474	2,955	1,359
Recorded investment with related allowance		250
Total recorded investment	2,474	3,205	1,359
Related Allowance		73
Average recorded investment	1,902	2,943	2,020
Interest income recognized	58	24	24
PNCI [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,388	1,458	1,644
Recorded investment with no related allowance	675	444	635
Recorded investment with related allowance	637	983	974
Total recorded investment	1,312	1,427	1,609
Related Allowance	394	498	467
Average recorded investment	1,453	1,451	1,543
Interest income recognized	29	41	65
PNCI [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		6
Related Allowance		6
Average recorded investment		3
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,302	1,634	1,404
Recorded investment with no related allowance	1,219	1,356	1,359
Recorded investment with related allowance		250
Total recorded investment	1,219	1,606	1,359
Related Allowance		73
Average recorded investment	1,275	1,165	1,041
Interest income recognized		24	24
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	296	185	178
Recorded investment with no related allowance	50	50	44
Recorded investment with related allowance	239	123	121
Total recorded investment	289	173	165
Related Allowance	145	99	97
Average recorded investment	287	121	117
Interest income recognized	9	5	6
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	240	253	250
Recorded investment with related allowance	240	253	250
Total recorded investment	240	253	250
Related Allowance	100	71	54
Average recorded investment	257	187	186
Interest income recognized	7	8	11
PNCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,255	1,869
Recorded investment with no related allowance	1,255	1,599
Total recorded investment	1,255	1,599
Average recorded investment	627	1,778	979
Interest income recognized	58
PNCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	852	1,020	1,216
Recorded investment with no related allowance	625	394	591
Recorded investment with related allowance	158	607	603
Total recorded investment	783	1,001	1,194
Related Allowance	149	328	316
Average recorded investment	909	1,143	1,240
Interest income recognized	$ 13	$ 28	$ 48