Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Originated [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 21,577	$ 25,693	$ 27,385
Recorded investment with no related allowance	13,537	17,455	20,904
Recorded investment with related allowance	6,122	6,528	4,539
Total recorded investment	19,659	23,983	25,443
Related Allowance	2,237	2,426	1,948
Average recorded investment	22,921	23,431	24,713
Interest income recognized	86	75	386
Originated [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	12,645	16,936	17,675
Recorded investment with no related allowance	8,808	13,710	14,339
Recorded investment with related allowance	2,765	2,126	2,073
Total recorded investment	11,573	15,836	16,412
Related Allowance	862	229	98
Average recorded investment	14,295	15,575	16,632
Interest income recognized	58	37	227
Originated [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,782	3,805	4,277
Recorded investment with no related allowance	2,845	2,936	3,641
Recorded investment with related allowance	322	460	179
Total recorded investment	3,167	3,396	3,820
Related Allowance	74	70	76
Average recorded investment	3,561	3,102	3,422
Interest income recognized	13	18	68
Originated [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	5,150	4,952	5,433
Recorded investment with no related allowance	1,884	809	2,924
Recorded investment with related allowance	3,035	3,942	2,287
Total recorded investment	4,919	4,751	5,211
Related Allowance	1,301	2,127	1,774
Average recorded investment	5,065	4,686	4,654
Interest income recognized	15	20	91
Originated [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment		68	5
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,739	5,656	4,594
Recorded investment with no related allowance	3,705	3,947	3,663
Recorded investment with related allowance	300	1,050	308
Total recorded investment	4,005	4,997	3,971
Related Allowance	54	147	56
Average recorded investment	4,291	4,600	3,517
Interest income recognized	18	28	90
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,373	2,558	2,374
Recorded investment with no related allowance	1,571	1,828	1,892
Recorded investment with related allowance	268	351	65
Total recorded investment	1,839	2,179	1,957
Related Allowance	61	38	2
Average recorded investment	2,003	1,784	1,520
Interest income recognized	4	15	23
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	76	3	3
Recorded investment with no related allowance	3
Recorded investment with related allowance	54	3	3
Total recorded investment	57	3	3
Related Allowance	13	3	3
Average recorded investment	48	3	17
Interest income recognized	1		2
Originated [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	7,906	11,280	13,081
Recorded investment with no related allowance	5,103	9,763	10,676
Recorded investment with related allowance	2,465	1,076	1,765
Total recorded investment	7,568	10,839	12,441
Related Allowance	808	82	42
Average recorded investment	10,004	10,975	13,115
Interest income recognized	40	9	137
Originated [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,333	1,244	1,900
Recorded investment with no related allowance	1,271	1,108	1,749
Recorded investment with related allowance		106	111
Total recorded investment	1,271	1,214	1,860
Related Allowance		29	71
Average recorded investment	1,510	1,315	1,885
Interest income recognized	8	3	43
Originated [Member] | Residential [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment		68	5
PNCI [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	6,372	3,375	4,990
Recorded investment with no related allowance	4,619	2,236	2,570
Recorded investment with related allowance	417	992	656
Total recorded investment	5,036	3,228	3,226
Related Allowance	205	463	246
Average recorded investment	4,156	3,075	3,691
Interest income recognized	171	11	207
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	5,418	1,740	3,485
Recorded investment with no related allowance	3,762	1,671	1,802
Total recorded investment	3,762	1,671	1,802
Average recorded investment	2,782	1,500	2,242
Interest income recognized	171	9	188
PNCI [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	841	1,635	1,385
Recorded investment with no related allowance	744	565	655
Recorded investment with related allowance	347	992	649
Total recorded investment	1,091	1,557	1,304
Related Allowance	135	463	239
Average recorded investment	1,223	1,575	1,389
Interest income recognized		2	18
PNCI [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	113		120
Recorded investment with no related allowance	113		113
Recorded investment with related allowance	70		7
Total recorded investment	183		120
Related Allowance	70		7
Average recorded investment	151		60
Interest income recognized			1
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	339	1,417	375
Recorded investment with no related allowance	300	1,348	334
Total recorded investment	300	1,348	334
Average recorded investment	317	1,339	529
Interest income recognized			5
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	167	293	252
Recorded investment with no related allowance	148	36	47
Recorded investment with related allowance	69	242	197
Total recorded investment	217	278	244
Related Allowance	44	154	101
Average recorded investment	229	281	155
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	62	244	106
Recorded investment with no related allowance	62		21
Recorded investment with related allowance	37	244	85
Total recorded investment	99	244	106
Related Allowance	6	51	11
Average recorded investment	105	259	114
PNCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	5,079	323	3,110
Recorded investment with no related allowance	3,462	323	1,468
Total recorded investment	3,462	323	1,468
Average recorded investment	2,465	161	1,713
Interest income recognized	171	9	183
PNCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	612	1,098	1,027
Recorded investment with no related allowance	534	529	587
Recorded investment with related allowance	241	506	367
Total recorded investment	775	1,035	954
Related Allowance	85	258	127
Average recorded investment	$ 889	1,035	1,120
Interest income recognized		$ 2	$ 18