Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Originated
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 18,737	$ 25,607	$ 27,385
Recorded investment with no related allowance	10,864	18,311	20,904
Recorded investment with related allowance	6,130	5,397	4,539
Total recorded investment	16,994	23,708	25,443
Related Allowance	1,376	2,234	1,948
Average recorded investment	21,760	23,292	24,713
Interest income recognized	145	409	386
Originated | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	10,467	16,738	17,675
Recorded investment with no related allowance	7,476	12,870	14,339
Recorded investment with related allowance	2,125	2,681	2,073
Total recorded investment	9,601	15,551	16,412
Related Allowance	117	325	98
Average recorded investment	13,310	15,432	16,632
Interest income recognized	91	275	227
Originated | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,510	4,001	4,277
Recorded investment with no related allowance	2,777	3,306	3,641
Recorded investment with related allowance	289	219	179
Total recorded investment	3,066	3,525	3,820
Related Allowance	60	52	76
Average recorded investment	3,681	3,166	3,422
Interest income recognized	32	56	68
Originated | Commercial
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,760	4,868	5,433
Recorded investment with no related allowance	611	2,135	2,924
Recorded investment with related allowance	3,716	2,497	2,287
Total recorded investment	4,327	4,632	5,211
Related Allowance	1,199	1,857	1,774
Average recorded investment	4,769	4,626	4,654
Interest income recognized	22	78	91
Originated | Construction
Financing Receivable, Impaired [Line Items]
Average recorded investment		68	5
Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,461	4,185	4,594
Recorded investment with no related allowance	3,053	3,251	3,663
Recorded investment with related allowance	796	311	308
Total recorded investment	3,849	3,562	3,971
Related Allowance	91	57	56
Average recorded investment	4,214	3,883	3,517
Interest income recognized	32	67	90
Originated | Home equity loans | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,157	2,554	2,374
Recorded investment with no related allowance	1,542	1,960	1,892
Recorded investment with related allowance	238	157	65
Total recorded investment	1,780	2,117	1,957
Related Allowance	46	30	2
Average recorded investment	1,973	1,753	1,520
Interest income recognized	5	24	23
Originated | Other | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	74	3	3
Recorded investment with no related allowance	3
Recorded investment with related allowance	51	3	3
Total recorded investment	54	3	3
Related Allowance	14	3	3
Average recorded investment	46	3	17
Interest income recognized	1		2
Originated | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	6,006	12,553	13,081
Recorded investment with no related allowance	4,423	9,619	10,676
Recorded investment with related allowance	1,329	2,370	1,765
Total recorded investment	5,752	11,989	12,441
Related Allowance	26	268	42
Average recorded investment	9,096	11,549	13,115
Interest income recognized	59	208	137
Originated | Home equity lines of credit | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,279	1,444	1,900
Recorded investment with no related allowance	1,232	1,346	1,749
Recorded investment with related allowance		59	111
Total recorded investment	1,232	1,405	1,860
Related Allowance		19	71
Average recorded investment	1,662	1,410	1,885
Interest income recognized	26	32	43
Originated | Residential | Construction
Financing Receivable, Impaired [Line Items]
Average recorded investment		68	5
PNCI
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	5,626	3,945	4,990
Recorded investment with no related allowance	5,166	3,149	2,570
Recorded investment with related allowance	324	637	656
Total recorded investment	5,490	3,786	3,226
Related Allowance	148	394	246
Average recorded investment	4,383	3,355	3,691
Interest income recognized	152	87	207
PNCI | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,214	2,557	3,485
Recorded investment with no related allowance	4,161	2,474	1,802
Total recorded investment	4,161	2,474	1,802
Average recorded investment	2,981	1,902	2,242
Interest income recognized	143	58	188
PNCI | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,231	1,388	1,385
Recorded investment with no related allowance	892	675	655
Recorded investment with related allowance	264	637	649
Total recorded investment	1,156	1,312	1,304
Related Allowance	88	394	239
Average recorded investment	1,255	1,453	1,389
Interest income recognized	9	29	18
PNCI | Commercial
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	181		120
Recorded investment with no related allowance	113		113
Recorded investment with related allowance	60		7
Total recorded investment	173		120
Related Allowance	60		7
Average recorded investment	147		60
Interest income recognized			1
PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	959	1,302	375
Recorded investment with no related allowance	908	1,219	334
Total recorded investment	908	1,219	334
Average recorded investment	621	1,275	529
Interest income recognized	9		5
PNCI | Home equity loans | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	395	296	252
Recorded investment with no related allowance	376	50	47
Recorded investment with related allowance	0	239	197
Total recorded investment	376	289	244
Related Allowance	0	145	101
Average recorded investment	308	287	155
Interest income recognized	9	9
PNCI | Other | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	95	240	106
Recorded investment with no related allowance	72		21
Recorded investment with related allowance	23	240	85
Total recorded investment	95	240	106
Related Allowance	4	100	11
Average recorded investment	103	257	114
Interest income recognized		7
PNCI | Commercial loans | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,255	1,255	3,110
Recorded investment with no related allowance	3,253	1,255	1,468
Total recorded investment	3,253	1,255	1,468
Average recorded investment	2,360	627	1,713
Interest income recognized	134	58	183
PNCI | Home equity lines of credit | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	741	852	1,027
Recorded investment with no related allowance	444	625	587
Recorded investment with related allowance	241	158	367
Total recorded investment	685	783	954
Related Allowance	84	149	127
Average recorded investment	$ 844	909	1,120
Interest income recognized		$ 13	$ 18