Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Originated [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	$ 19,693	$ 23,903	$ 27,385
Recorded investment with no related allowance	15,032	15,624	20,904
Recorded investment with related allowance	2,841	7,254	4,539
Total recorded investment	17,873	22,878	25,443
Related Allowance	1,362	2,617	1,948
Average recorded investment	22,199	24,072	24,713
Interest income recognized	32	112	386
Originated [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	10,919	15,785	17,675
Recorded investment with no related allowance	9,355	12,526	14,339
Recorded investment with related allowance	592	2,787	2,073
Total recorded investment	9,947	15,313	16,412
Related Allowance	92	344	98
Average recorded investment	13,482	16,581	16,632
Interest income recognized	28	72	227
Originated [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,236	3,226	4,277
Recorded investment with no related allowance	3,376	2,081	3,641
Recorded investment with related allowance	206	727	179
Total recorded investment	3,582	2,808	3,820
Related Allowance	47	160	76
Average recorded investment	3,939	2,808	3,422
Interest income recognized	4	12	68
Originated [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,538	4,756	5,433
Recorded investment with no related allowance	2,301	881	2,924
Recorded investment with related allowance	2,043	3,740	2,287
Total recorded investment	4,344	4,621	5,211
Related Allowance	1,223	2,113	1,774
Average recorded investment	4,778	4,545	4,654
Interest income recognized		26	91
Originated [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		136
Recorded investment with no related allowance		136
Total recorded investment		136
Average recorded investment		138	5
Interest income recognized		2
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,148	4,378	4,594
Recorded investment with no related allowance	3,481	2,678	3,663
Recorded investment with related allowance		1,525	308
Total recorded investment	3,481	4,203	3,971
Related Allowance	55	190	56
Average recorded investment	4,029	4,071	3,517
Interest income recognized	6	28	90
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	2,333	1,744	2,374
Recorded investment with no related allowance	1,639	1,193	1,892
Recorded investment with related allowance	120	196	65
Total recorded investment	1,759	1,389	1,957
Related Allowance	20	10	2
Average recorded investment	1,963	1,347	1,520
Interest income recognized		2	23
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	46	4	3
Recorded investment with related allowance	28	4	3
Total recorded investment	28	4	3
Related Allowance	9	4	3
Average recorded investment	33	6	17
Interest income recognized			2
Originated [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	6,771	11,407	13,081
Recorded investment with no related allowance	5,874	9,848	10,676
Recorded investment with related allowance	592	1,262	1,765
Total recorded investment	6,466	11,110	12,441
Related Allowance	37	154	42
Average recorded investment	9,453	12,510	13,115
Interest income recognized	22	44	137
Originated [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,857	1,478	1,900
Recorded investment with no related allowance	1,737	888	1,749
Recorded investment with related allowance	58	527	111
Total recorded investment	1,795	1,415	1,860
Related Allowance	18	146	71
Average recorded investment	1,943	1,455	1,885
Interest income recognized	4	10	43
Originated [Member] | Residential [Member] | Construction [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance		136
Recorded investment with no related allowance		136
Total recorded investment		136
Average recorded investment		138	5
Interest income recognized		2
PNCI [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	4,943	3,027	4,990
Recorded investment with no related allowance	2,433	1,901	2,570
Recorded investment with related allowance	739	1,024	656
Total recorded investment	3,172	2,925	3,226
Related Allowance	388	474	246
Average recorded investment	3,224	2,946	3,691
Interest income recognized	60	12	207
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,433	1,390	3,485
Recorded investment with no related allowance	1,753	1,332	1,802
Total recorded investment	1,753	1,332	1,802
Average recorded investment	1,777	1,345	2,242
Interest income recognized	58	2	188
PNCI [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	1,175	1,637	1,385
Recorded investment with no related allowance	567	569	655
Recorded investment with related allowance	520	1,024	649
Total recorded investment	1,087	1,593	1,304
Related Allowance	169	474	239
Average recorded investment	1,221	1,601	1,389
Interest income recognized		10	18
PNCI [Member] | Commercial Portfolio Segment [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	335		120
Recorded investment with no related allowance	113		113
Recorded investment with related allowance	219		7
Total recorded investment	332		120
Related Allowance	219		7
Average recorded investment	226		60
Interest income recognized	2		1
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	344	1,390	375
Recorded investment with no related allowance	308	1,332	334
Total recorded investment	308	1,332	334
Average recorded investment	321	1,345	529
Interest income recognized		2	5
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	242	298	252
Recorded investment with no related allowance	102	40	47
Recorded investment with related allowance	122	244	197
Total recorded investment	224	284	244
Related Allowance	50	120	101
Average recorded investment	232	225	155
Interest income recognized		3
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	102	274	106
Recorded investment with no related allowance	64	28	21
Recorded investment with related allowance	38	246	85
Total recorded investment	102	274	106
Related Allowance	7	52	11
Average recorded investment	106	262	114
Interest income recognized		2
PNCI [Member] | Commercial [Member] | Mortgage Loans on Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	3,089		3,110
Recorded investment with no related allowance	1,445		1,468
Total recorded investment	1,445		1,468
Average recorded investment	1,456		1,713
Interest income recognized	58		183
PNCI [Member] | Home Equity Lines of Credit [Member] | Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid principal balance	831	1,065	1,027
Recorded investment with no related allowance	401	501	587
Recorded investment with related allowance	360	534	367
Total recorded investment	761	1,035	954
Related Allowance	112	302	127
Average recorded investment	$ 883	1,114	1,120
Interest income recognized		$ 5	$ 18