Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	$ 18,078	$ 17,701
Unpaid principal, With no related allowance recorded	18,850	18,641
Average recorded Investment, With no related allowance recorded	17,891	26,601
Interest income Recognized, With no related allowance recorded	774	757
Recorded investment, With an allowance recorded	7,188	6,285
Unpaid principal, With an allowance recorded	7,328	6,349
Related allowance, With an allowance recorded	2,232	1,544
Average recorded Investment, With an allowance recorded	6,735	7,100
Interest income Recognized, With an allowance recorded	278	206
PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,994	3,094
Unpaid principal, With no related allowance recorded	2,073	3,341
Average recorded Investment, With no related allowance recorded	2,545	3,075
Interest income Recognized, With no related allowance recorded	46	17
Recorded investment, With an allowance recorded	974	1,060
Unpaid principal, With an allowance recorded	975	1,060
Related allowance, With an allowance recorded	467	502
Average recorded Investment, With an allowance recorded	1,018	2,344
Interest income Recognized, With an allowance recorded	43	49
Mortgage Loans on Real Estate [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	2,058	1,820
Unpaid principal, With no related allowance recorded	2,109	1,829
Average recorded Investment, With no related allowance recorded	1,875	2,853
Interest income Recognized, With no related allowance recorded	85	92
Recorded investment, With an allowance recorded	1,881	1,551
Unpaid principal, With an allowance recorded	1,914	1,552
Related allowance, With an allowance recorded	230	180
Average recorded Investment, With an allowance recorded	1,626	1,779
Interest income Recognized, With an allowance recorded	58	65
Mortgage Loans on Real Estate [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,359	463
Unpaid principal, With no related allowance recorded	1,404	486
Average recorded Investment, With no related allowance recorded	911	669
Interest income Recognized, With no related allowance recorded	24	7
Recorded investment, With an allowance recorded		259
Unpaid principal, With an allowance recorded		259
Related allowance, With an allowance recorded		79
Average recorded Investment, With an allowance recorded	130	130
Interest income Recognized, With an allowance recorded		10
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	13,101	12,898
Unpaid principal, With no related allowance recorded	13,360	13,145
Average recorded Investment, With no related allowance recorded	13,123	20,003
Interest income Recognized, With no related allowance recorded	609	570
Recorded investment, With an allowance recorded	810	357
Unpaid principal, With an allowance recorded	826	358
Related allowance, With an allowance recorded	30	4
Average recorded Investment, With an allowance recorded	728	888
Interest income Recognized, With an allowance recorded	36	22
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded		1,826
Unpaid principal, With no related allowance recorded		2,031
Average recorded Investment, With no related allowance recorded	913	1,479
Average recorded Investment, With an allowance recorded	66	1,374
Consumer [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,093	1,480
Unpaid principal, With no related allowance recorded	1,175	1,561
Average recorded Investment, With no related allowance recorded	1,287	2,221
Interest income Recognized, With no related allowance recorded	39	40
Recorded investment, With an allowance recorded	401	430
Unpaid principal, With an allowance recorded	406	440
Related allowance, With an allowance recorded	111	110
Average recorded Investment, With an allowance recorded	415	1,076
Interest income Recognized, With an allowance recorded	8	9
Consumer [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	591	735
Unpaid principal, With no related allowance recorded	612	746
Average recorded Investment, With no related allowance recorded	663	594
Interest income Recognized, With no related allowance recorded	22	9
Recorded investment, With an allowance recorded	603	551
Unpaid principal, With an allowance recorded	604	551
Related allowance, With an allowance recorded	316	300
Average recorded Investment, With an allowance recorded	577	579
Interest income Recognized, With an allowance recorded	26	27
Consumer [Member] | Home Equity Loans [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,107	715
Unpaid principal, With no related allowance recorded	1,429	1,135
Average recorded Investment, With no related allowance recorded	852	831
Interest income Recognized, With no related allowance recorded	14	6
Recorded investment, With an allowance recorded	198	594
Unpaid principal, With an allowance recorded	198	595
Related allowance, With an allowance recorded	10	107
Average recorded Investment, With an allowance recorded	341	634
Interest income Recognized, With an allowance recorded	10	31
Consumer [Member] | Home Equity Loans [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	44	67
Unpaid principal, With no related allowance recorded	57	74
Average recorded Investment, With no related allowance recorded	56	69
Interest income Recognized, With no related allowance recorded		1
Recorded investment, With an allowance recorded	121	132
Unpaid principal, With an allowance recorded	121	132
Related allowance, With an allowance recorded	97	108
Average recorded Investment, With an allowance recorded	61	85
Interest income Recognized, With an allowance recorded	6	7
Consumer [Member] | Other Consumer [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	4	15
Unpaid principal, With no related allowance recorded	52	29
Average recorded Investment, With no related allowance recorded	10	16
Interest income Recognized, With no related allowance recorded		1
Recorded investment, With an allowance recorded	3	19
Unpaid principal, With an allowance recorded	3	19
Related allowance, With an allowance recorded	3	13
Average recorded Investment, With an allowance recorded	10	9
Interest income Recognized, With an allowance recorded		2
Consumer [Member] | Other Consumer [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded		3
Unpaid principal, With no related allowance recorded		4
Average recorded Investment, With no related allowance recorded	2	18
Recorded investment, With an allowance recorded	250	118
Unpaid principal, With an allowance recorded	250	118
Related allowance, With an allowance recorded	54	15
Average recorded Investment, With an allowance recorded	184	176
Interest income Recognized, With an allowance recorded	11	5
Consumer [Member] | Auto Indirect [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded Investment, With no related allowance recorded		1
Consumer and Industrial [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	575	762
Unpaid principal, With no related allowance recorded	585	926
Average recorded Investment, With no related allowance recorded	668	669
Interest income Recognized, With no related allowance recorded	18	48
Recorded investment, With an allowance recorded	3,895	3,334
Unpaid principal, With an allowance recorded	3,981	3,385
Related allowance, With an allowance recorded	1,848	1,130
Average recorded Investment, With an allowance recorded	3,615	2,714
Interest income Recognized, With an allowance recorded	166	77
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	140	11
Unpaid principal, With no related allowance recorded	140	16
Average recorded Investment, With no related allowance recorded	76	7
Interest income Recognized, With no related allowance recorded	$ 9
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded Investment, With no related allowance recorded		$ 245