Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Originated [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 27,385	$ 26,178	$ 24,990
Recorded investment with no related allowance	20,904	18,078	17,701
Recorded investment with related allowance	4,539	7,188	6,285
Total recorded investment	25,443	25,266	23,986
Related Allowance	1,948	2,232	1,544
Average recorded investment	24,713	24,626	33,701
Interest income recognized	386	1,052	963
Originated [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	17,675	18,209	16,884
Recorded investment with no related allowance	14,339	15,159	14,718
Recorded investment with related allowance	2,073	2,691	1,908
Total recorded investment	16,412	17,850	16,626
Related Allowance	98	260	184
Average recorded investment	16,632	17,352	25,523
Interest income recognized	227	788	749
Originated [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,277	3,263	3,779
Recorded investment with no related allowance	3,641	2,204	2,210
Recorded investment with related allowance	179	602	1,043
Total recorded investment	3,820	2,806	3,253
Related Allowance	76	124	230
Average recorded investment	3,422	2,915	4,788
Interest income recognized	68	71	89
Originated [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	5,433	4,566	4,311
Recorded investment with no related allowance	2,924	575	762
Recorded investment with related allowance	2,287	3,895	3,334
Total recorded investment	5,211	4,470	4,096
Related Allowance	1,774	1,848	1,130
Average recorded investment	4,654	4,283	3,383
Interest income recognized	91	184	125
Originated [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		140	16
Recorded investment with no related allowance		140	11
Total recorded investment		140	11
Average recorded investment	5	76	7
Interest income recognized		9
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,594	4,023	3,381
Recorded investment with no related allowance	3,663	2,058	1,820
Recorded investment with related allowance	308	1,881	1,551
Total recorded investment	3,971	3,939	3,371
Related Allowance	56	230	180
Average recorded investment	3,517	3,501	4,632
Interest income recognized	90	143	157
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,374	1,627	1,730
Recorded investment with no related allowance	1,892	1,107	715
Recorded investment with related allowance	65	198	594
Total recorded investment	1,957	1,305	1,309
Related Allowance	2	10	107
Average recorded investment	1,520	1,193	1,465
Interest income recognized	23	24	37
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3	55	48
Recorded investment with no related allowance		4	15
Recorded investment with related allowance	3	3	19
Total recorded investment	3	7	34
Related Allowance	3	3	13
Average recorded investment	17	20	26
Interest income recognized	2		3
Originated [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	13,081	14,186	13,503
Recorded investment with no related allowance	10,676	13,101	12,898
Recorded investment with related allowance	1,765	810	357
Total recorded investment	12,441	13,911	13,255
Related Allowance	42	30	4
Average recorded investment	13,115	13,851	20,891
Interest income recognized	137	645	592
Originated [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,900	1,581	2,001
Recorded investment with no related allowance	1,749	1,093	1,480
Recorded investment with related allowance	111	401	430
Total recorded investment	1,860	1,494	1,910
Related Allowance	71	111	110
Average recorded investment	1,885	1,702	3,297
Interest income recognized	43	47	49
Originated [Member] | Residential [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		140	16
Recorded investment with no related allowance		140	11
Total recorded investment		140	11
Average recorded investment	5	76	7
Interest income recognized		9
PNCI [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,990	3,048	4,401
Recorded investment with no related allowance	2,570	1,994	3,094
Recorded investment with related allowance	656	974	1,060
Total recorded investment	3,226	2,968	4,154
Related Allowance	246	467	502
Average recorded investment	3,691	3,563	5,419
Interest income recognized	207	89	66
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,485	1,404	2,776
Recorded investment with no related allowance	1,802	1,359	2,289
Recorded investment with related allowance			259
Total recorded investment	1,802	1,359	2,548
Related Allowance			79
Average recorded investment	2,242	2,020	3,652
Interest income recognized	188	24	17
PNCI [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,385	1,644	1,625
Recorded investment with no related allowance	655	635	805
Recorded investment with related allowance	649	974	801
Total recorded investment	1,304	1,609	1,606
Related Allowance	239	467	423
Average recorded investment	1,389	1,543	1,521
Interest income recognized	18	65	49
PNCI [Member] | Commercial Portfolio Segment [Member]
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		1
Interest income recognized	1
PNCI [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment			245
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	375	1,404	745
Recorded investment with no related allowance	334	1,359	463
Recorded investment with related allowance			259
Total recorded investment	334	1,359	722
Related Allowance			79
Average recorded investment	529	1,041	799
Interest income recognized	5	24	17
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	252	178	206
Recorded investment with no related allowance	47	44	67
Recorded investment with related allowance	197	121	132
Total recorded investment	244	165	199
Related Allowance	101	97	108
Average recorded investment	155	117	154
Interest income recognized		6	8
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	106	250	122
Recorded investment with no related allowance	21		3
Recorded investment with related allowance	85	250	118
Total recorded investment	106	250	121
Related Allowance	11	54	15
Average recorded investment	114	186	194
Interest income recognized		11	5
PNCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,110		2,031
Recorded investment with no related allowance	1,468		1,826
Total recorded investment	1,468		1,826
Average recorded investment	1,713	979	2,853
Interest income recognized	183
PNCI [Member] | Commercial [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment			245
PNCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,027	1,216	1,297
Recorded investment with no related allowance	587	591	735
Recorded investment with related allowance	367	603	551
Total recorded investment	954	1,194	1,286
Related Allowance	127	316	300
Average recorded investment	1,120	1,240	1,173
Interest income recognized	$ 18	$ 48	$ 36