Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	$ 15,624	$ 19,325	$ 18,078
Unpaid principal, With no related allowance recorded	16,477	20,237	18,850
Average recorded investment, With no related allowance recorded	16,851	27,413	17,891
Interest income recognized, With no related allowance recorded	71	132	774
Recorded investment, With an allowance recorded	7,254	4,703	7,188
Unpaid principal, With an allowance recorded	7,426	4,783	7,328
Related allowance, With an allowance recorded	2,617	1,192	2,232
Average recorded investment, With an allowance recorded	7,221	6,310	6,735
Interest income recognized, With an allowance recorded	41	40	278
PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,901	2,626	1,994
Unpaid principal, With no related allowance recorded	2,001	2,905	2,073
Average recorded investment, With no related allowance recorded	1,947	2,842	2,545
Interest income recognized, With no related allowance recorded	4	5	46
Recorded investment, With an allowance recorded	1,024	996	974
Unpaid principal, With an allowance recorded	1,026	996	975
Related allowance, With an allowance recorded	474	494	467
Average recorded investment, With an allowance recorded	999	2,312	1,018
Interest income recognized, With an allowance recorded	8	10	43
Mortgage Loans on Real Estate [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	2,678	1,782	2,058
Unpaid principal, With no related allowance recorded	2,829	1,793	2,109
Average recorded investment, With no related allowance recorded	2,368	2,834	1,875
Interest income recognized, With no related allowance recorded	19	20	85
Recorded investment, With an allowance recorded	1,525	1,378	1,881
Unpaid principal, With an allowance recorded	1,549	1,382	1,914
Related allowance, With an allowance recorded	190	172	230
Average recorded investment, With an allowance recorded	1,703	1,692	1,626
Interest income recognized, With an allowance recorded	9	11	58
Mortgage Loans on Real Estate [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,332	433	1,359
Unpaid principal, With no related allowance recorded	1,390	455	1,404
Average recorded investment, With no related allowance recorded	1,345	654	911
Interest income recognized, With no related allowance recorded	2	2	24
Recorded investment, With an allowance recorded		256
Unpaid principal, With an allowance recorded		256
Related allowance, With an allowance recorded		77
Average recorded investment, With an allowance recorded		128	130
Interest income recognized, With an allowance recorded		2
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	9,848	14,431	13,101
Unpaid principal, With no related allowance recorded	10,126	14,881	13,360
Average recorded investment, With no related allowance recorded	11,474	20,770	13,123
Interest income recognized, With no related allowance recorded	35	98	609
Recorded investment, With an allowance recorded	1,262	640	810
Unpaid principal, With an allowance recorded	1,281	640	826
Related allowance, With an allowance recorded	154	18	30
Average recorded investment, With an allowance recorded	1,036	1,029	728
Interest income recognized, With an allowance recorded	9	9	36
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded		1,777
Unpaid principal, With no related allowance recorded		2,011
Average recorded investment, With no related allowance recorded		1,455	913
Recorded investment, With an allowance recorded		131
Unpaid principal, With an allowance recorded		131
Related allowance, With an allowance recorded		106
Average recorded investment, With an allowance recorded		1,440	66
Interest income recognized, With an allowance recorded		2
Consumer [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	888	1,064	1,093
Unpaid principal, With no related allowance recorded	944	1,144	1,175
Average recorded investment, With no related allowance recorded	991	2,013	1,287
Interest income recognized, With no related allowance recorded	6	6	39
Recorded investment, With an allowance recorded	527	427	401
Unpaid principal, With an allowance recorded	534	440	406
Related allowance, With an allowance recorded	146	106	111
Average recorded investment, With an allowance recorded	464	1,076	415
Interest income recognized, With an allowance recorded	4	1	8
Consumer [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	501	220	591
Unpaid principal, With no related allowance recorded	529	233	612
Average recorded investment, With no related allowance recorded	546	337	663
Interest income recognized, With no related allowance recorded	2	1	22
Recorded investment, With an allowance recorded	534	493	603
Unpaid principal, With an allowance recorded	536	493	604
Related allowance, With an allowance recorded	302	295	316
Average recorded investment, With an allowance recorded	568	550	577
Interest income recognized, With an allowance recorded	3	5	26
Consumer [Member] | Home Equity Loans [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,193	743	1,107
Unpaid principal, With no related allowance recorded	1,548	1,085	1,429
Average recorded investment, With no related allowance recorded	1,150	845	852
Interest income recognized, With no related allowance recorded		1	14
Recorded investment, With an allowance recorded	196	440	198
Unpaid principal, With an allowance recorded	196	443	198
Related allowance, With an allowance recorded	10	57	10
Average recorded investment, With an allowance recorded	197	557	341
Interest income recognized, With an allowance recorded	2	5	10
Consumer [Member] | Home Equity Loans [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	40	58	44
Unpaid principal, With no related allowance recorded	54	67	57
Average recorded investment, With no related allowance recorded	42	64	56
Recorded investment, With an allowance recorded	244		121
Unpaid principal, With an allowance recorded	244		121
Related allowance, With an allowance recorded	120		97
Average recorded investment, With an allowance recorded	183	19	61
Interest income recognized, With an allowance recorded	3		6
Consumer [Member] | Other Consumer [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded		5	4
Unpaid principal, With no related allowance recorded		6	52
Average recorded investment, With no related allowance recorded	2	12	10
Recorded investment, With an allowance recorded	4	21	3
Unpaid principal, With an allowance recorded	4	22	3
Related allowance, With an allowance recorded	4	14	3
Average recorded investment, With an allowance recorded	4	11	10
Consumer [Member] | Other Consumer [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	28	138
Unpaid principal, With no related allowance recorded	28	139
Average recorded investment, With no related allowance recorded	14	86	2
Interest income recognized, With no related allowance recorded		2
Recorded investment, With an allowance recorded	246	116	250
Unpaid principal, With an allowance recorded	246	116	250
Related allowance, With an allowance recorded	52	16	54
Average recorded investment, With an allowance recorded	248	175	184
Interest income recognized, With an allowance recorded	2	1	11
Consumer and Industrial [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	881	1,289	575
Unpaid principal, With no related allowance recorded	894	1,312	585
Average recorded investment, With no related allowance recorded	728	932	668
Interest income recognized, With no related allowance recorded	9	7	18
Recorded investment, With an allowance recorded	3,740	1,783	3,895
Unpaid principal, With an allowance recorded	3,862	1,842	3,981
Related allowance, With an allowance recorded	2,113	811	1,848
Average recorded investment, With an allowance recorded	3,817	1,938	3,615
Interest income recognized, With an allowance recorded	17	14	166
Consumer and Industrial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment, With no related allowance recorded		1
Construction [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	136	11	140
Unpaid principal, With no related allowance recorded	136	16	140
Average recorded investment, With no related allowance recorded	138	7	76
Interest income recognized, With no related allowance recorded	$ 2		$ 9
Recorded investment, With an allowance recorded		14
Unpaid principal, With an allowance recorded		14
Related allowance, With an allowance recorded		14
Average recorded investment, With an allowance recorded		7
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded investment, With no related allowance recorded		$ 245