Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		$ 120
Recorded investment with no related allowance		113
Recorded investment with related allowance		7
Total recorded investment		120
Related allowance		7
Average recorded investment		60
Interest income recognized		1
Originated
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 17,248	27,385	$ 26,178
Recorded investment with no related allowance	11,846	20,904	18,078
Recorded investment with related allowance	3,685	4,539	7,188
Total recorded investment	15,531	25,443	25,266
Related allowance	756	1,948	2,232
Average recorded investment	21,028	24,713	24,626
Interest income recognized	211	386	1,052
Originated | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	11,379	17,675	18,209
Recorded investment with no related allowance	7,835	14,339	15,159
Recorded investment with related allowance	2,609	2,073	2,691
Total recorded investment	10,444	16,412	17,850
Related allowance	127	98	260
Average recorded investment	13,731	16,632	17,352
Interest income recognized	157	227	788
Originated | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,257	4,277	3,263
Recorded investment with no related allowance	2,548	3,641	2,204
Recorded investment with related allowance	260	179	602
Total recorded investment	2,808	3,820	2,806
Related allowance	50	76	124
Average recorded investment	3,551	3,422	2,915
Interest income recognized	42	68	71
Originated | Commercial
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		5,433	4,566
Recorded investment with no related allowance		2,924	575
Recorded investment with related allowance		2,287	3,895
Total recorded investment		5,211	4,470
Related allowance		1,774	1,848
Average recorded investment		4,654	4,283
Interest income recognized		91	184
Originated | Construction
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0	0	140
Recorded investment with no related allowance	0	0	140
Recorded investment with related allowance	0
Total recorded investment	0	0	140
Related allowance	0
Average recorded investment	0	5	76
Interest income recognized	0	0	9
Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,836	4,594	4,023
Recorded investment with no related allowance	3,434	3,663	2,058
Recorded investment with related allowance	764	308	1,881
Total recorded investment	4,198	3,971	3,939
Related allowance	49	56	230
Average recorded investment	4,388	3,517	3,501
Interest income recognized	71	90	143
Originated | Home equity loans | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,895	2,374	1,627
Recorded investment with no related allowance	1,278	1,892	1,107
Recorded investment with related allowance	234	65	198
Total recorded investment	1,512	1,957	1,305
Related allowance	42	2	10
Average recorded investment	1,839	1,520	1,193
Interest income recognized	5	23	24
Originated | Other | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	47	3	55
Recorded investment with no related allowance	3	0	4
Recorded investment with related allowance	25	3	3
Total recorded investment	28	3	7
Related allowance	7	3	3
Average recorded investment	33	17	20
Interest income recognized	2	2	0
Originated | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	6,543	13,081	14,186
Recorded investment with no related allowance	4,401	10,676	13,101
Recorded investment with related allowance	1,845	1,765	810
Total recorded investment	6,246	12,441	13,911
Related allowance	78	42	30
Average recorded investment	9,343	13,115	13,851
Interest income recognized	86	137	645
Originated | Commercial | Commercial
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,612
Recorded investment with no related allowance	1,463
Recorded investment with related allowance	816
Total recorded investment	2,279
Related allowance	579
Average recorded investment	3,746
Interest income recognized	12
Originated | Home equity lines of credit | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,315	1,900	1,581
Recorded investment with no related allowance	1,267	1,749	1,093
Recorded investment with related allowance	1	111	401
Total recorded investment	1,268	1,860	1,494
Related allowance	1	71	111
Average recorded investment	1,679	1,885	1,702
Interest income recognized	35	43	47
Originated | Residential | Construction
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0	0	140
Recorded investment with no related allowance	0	0	140
Recorded investment with related allowance	0
Total recorded investment	0	0	140
Related allowance	0
Average recorded investment	0	5	76
Interest income recognized	0	0	9
PNCI
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,542	4,990	3,048
Recorded investment with no related allowance	3,753	2,570	1,994
Recorded investment with related allowance	633	656	974
Total recorded investment	4,386	3,226	2,968
Related allowance	179	246	467
Average recorded investment	3,831	3,691	3,563
Interest income recognized	161	207	89
PNCI | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,350	3,485	1,404
Recorded investment with no related allowance	3,279	1,802	1,359
Recorded investment with related allowance	0		0
Total recorded investment	3,279	1,802	1,359
Related allowance	0		0
Average recorded investment	2,540	2,242	2,020
Interest income recognized	155	188	24
PNCI | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,133	1,385	1,644
Recorded investment with no related allowance	474	655	635
Recorded investment with related allowance	582	649	974
Total recorded investment	1,056	1,304	1,609
Related allowance	128	239	467
Average recorded investment	1,205	1,389	1,543
Interest income recognized	6	18	65
PNCI | Construction
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0
Recorded investment with no related allowance	0
Recorded investment with related allowance	0
Total recorded investment	0
Related allowance	0
Average recorded investment	0
Interest income recognized	0
PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	945	375	1,404
Recorded investment with no related allowance	876	334	1,359
Recorded investment with related allowance	0		0
Total recorded investment	876	334	1,359
Related allowance	0		0
Average recorded investment	605	529	1,041
Interest income recognized	9	5	24
PNCI | Home equity loans | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	159	252	178
Recorded investment with no related allowance	20	47	44
Recorded investment with related allowance	118	197	121
Total recorded investment	138	244	165
Related allowance	2	101	97
Average recorded investment	189	155	117
Interest income recognized	0	0	6
PNCI | Other
Financing Receivable, Impaired [Line Items]
Recorded investment with no related allowance		21
PNCI | Other | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	112	106	250
Recorded investment with no related allowance	59		0
Recorded investment with related allowance	53	85	250
Total recorded investment	112	106	250
Related allowance	12	11	54
Average recorded investment	111	114	186
Interest income recognized	0	0	11
PNCI | Commercial | Mortgage Loans on Real Estate
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,405	3,110
Recorded investment with no related allowance	2,403	1,468
Recorded investment with related allowance	0
Total recorded investment	2,403	1,468
Related allowance	0
Average recorded investment	1,935	1,713	979
Interest income recognized	146	183
PNCI | Commercial | Commercial
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	59
Recorded investment with no related allowance	0
Recorded investment with related allowance	51
Total recorded investment	51
Related allowance	51
Average recorded investment	86
Interest income recognized	0
PNCI | Home equity lines of credit | Consumer
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	862	1,027	1,216
Recorded investment with no related allowance	395	587	591
Recorded investment with related allowance	411	367	603
Total recorded investment	806	954	1,194
Related allowance	114	127	316
Average recorded investment	905	1,120	1,240
Interest income recognized	6	$ 18	$ 48
PNCI | Residential | Construction
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	0
Recorded investment with no related allowance	0
Recorded investment with related allowance	0
Total recorded investment	0
Related allowance	0
Average recorded investment	0
Interest income recognized	$ 0