Allowance for Loan Losses - Impaired Loans (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	$ 22,425	$ 13,243	$ 17,701
Unpaid principal, With no related allowance recorded	23,743	15,442	18,641
Average recorded Investment, With no related allowance recorded	20,064	24,372	26,601
Interest income Recognized, With no related allowance recorded	573	356	757
Recorded investment, With an allowance recorded	5,162	12,246	6,285
Unpaid principal, With an allowance recorded	5,269	12,467	6,349
Related allowance, With an allowance recorded	1,563	2,314	1,544
Average recorded Investment, With an allowance recorded	5,723	10,082	7,100
Interest income Recognized, With an allowance recorded	139	391	206
PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	3,399	2,953	3,094
Unpaid principal, With no related allowance recorded	3,728	3,223	3,341
Average recorded Investment, With no related allowance recorded	3,247	3,004	3,075
Interest income Recognized, With no related allowance recorded	25	10	17
Recorded investment, With an allowance recorded	1,239	1,161	1,060
Unpaid principal, With an allowance recorded	1,239	1,161	1,060
Related allowance, With an allowance recorded	577	507	502
Average recorded Investment, With an allowance recorded	1,150	2,394	2,344
Interest income Recognized, With an allowance recorded	40	36	49
Mortgage Loans on Real Estate [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,777	2,392	1,821
Unpaid principal, With no related allowance recorded	1,819	3,083	1,829
Average recorded Investment, With no related allowance recorded	1,734	3,139	2,853
Interest income Recognized, With no related allowance recorded	51	63	92
Recorded investment, With an allowance recorded	1,644	2,613	1,551
Unpaid principal, With an allowance recorded	1,670	2,701	1,552
Related allowance, With an allowance recorded	167	381	180
Average recorded Investment, With an allowance recorded	1,508	2,309	1,779
Interest income Recognized, With an allowance recorded	39	68	65
Mortgage Loans on Real Estate [Member] | Residential [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,356	474	463
Unpaid principal, With no related allowance recorded	1,384	492	486
Average recorded Investment, With no related allowance recorded	910	674	669
Interest income Recognized, With no related allowance recorded	17	7	7
Recorded investment, With an allowance recorded	250	263	259
Unpaid principal, With an allowance recorded	250	263	259
Related allowance, With an allowance recorded	73	81	79
Average recorded Investment, With an allowance recorded	255	131	130
Interest income Recognized, With an allowance recorded	7	7	10
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	17,039	7,011	12,897
Unpaid principal, With no related allowance recorded	17,493	7,286	13,145
Average recorded Investment, With no related allowance recorded	15,092	17,059	20,003
Interest income Recognized, With no related allowance recorded	474	232	570
Recorded investment, With an allowance recorded	1,150	5,820	357
Unpaid principal, With an allowance recorded	1,150	5,843	357
Related allowance, With an allowance recorded	73	644	4
Average recorded Investment, With an allowance recorded	898	3,620	888
Interest income Recognized, With an allowance recorded	40	216	22
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,599	1,874	1,826
Unpaid principal, With no related allowance recorded	1,869	2,048	2,031
Average recorded Investment, With no related allowance recorded	1,712	1,503	1,479
Average recorded Investment, With an allowance recorded	66	1,374	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	1,108	1,888	1,480
Unpaid principal, With no related allowance recorded	1,209	2,318	1,561
Average recorded Investment, With no related allowance recorded	1,294	2,425	2,221
Interest income Recognized, With no related allowance recorded	25	26	40
Recorded investment, With an allowance recorded	110	923	430
Unpaid principal, With an allowance recorded	115	931	440
Related allowance, With an allowance recorded	33	263	110
Average recorded Investment, With an allowance recorded	270	1,324	1,076
Interest income Recognized, With an allowance recorded		16	9
Consumer [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	394	502	735
Unpaid principal, With no related allowance recorded	413	573	746
Average recorded Investment, With no related allowance recorded	564	478	594
Interest income Recognized, With no related allowance recorded	8	2	9
Recorded investment, With an allowance recorded	607	507	551
Unpaid principal, With an allowance recorded	607	507	551
Related allowance, With an allowance recorded	328	248	300
Average recorded Investment, With an allowance recorded	579	557	579
Interest income Recognized, With an allowance recorded	20	16	27
Consumer [Member] | Home Equity Loans [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	1,470	1,360	715
Unpaid principal, With no related allowance recorded	1,892	1,979	1,135
Average recorded Investment, With no related allowance recorded	1,034	1,154	831
Interest income Recognized, With no related allowance recorded	25	9	6
Recorded investment, With an allowance recorded	199	1,059	594
Unpaid principal, With an allowance recorded	199	1,115	596
Related allowance, With an allowance recorded	13	201	107
Average recorded Investment, With an allowance recorded	342	866	634
Interest income Recognized, With an allowance recorded	7	36	31
Consumer [Member] | Home Equity Loans [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	50	97	67
Unpaid principal, With no related allowance recorded	62	103	74
Average recorded Investment, With no related allowance recorded	59	84	69
Interest income Recognized, With no related allowance recorded		1	1
Recorded investment, With an allowance recorded	123	134	132
Unpaid principal, With an allowance recorded	123	134	132
Related allowance, With an allowance recorded	99	109	108
Average recorded Investment, With an allowance recorded	62	86	85
Interest income Recognized, With an allowance recorded	5	5	7
Consumer [Member] | Other Consumer [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	3	7	15
Unpaid principal, With no related allowance recorded	47	11	29
Average recorded Investment, With no related allowance recorded	9	12	17
Interest income Recognized, With no related allowance recorded	(25)		1
Recorded investment, With an allowance recorded	11	8	19
Unpaid principal, With an allowance recorded	12	8	19
Related allowance, With an allowance recorded	7	3	13
Average recorded Investment, With an allowance recorded	14	4	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		6	3
Unpaid principal, With no related allowance recorded		7	4
Average recorded Investment, With no related allowance recorded	2	19	18
Recorded investment, With an allowance recorded	253	257	118
Unpaid principal, With an allowance recorded	253	257	118
Related allowance, With an allowance recorded	71	69	15
Average recorded Investment, With an allowance recorded	185	246	176
Interest income Recognized, With an allowance recorded	8	8	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	884	574	762
Unpaid principal, With no related allowance recorded	1,139	749	926
Average recorded Investment, With no related allowance recorded	823	575	669
Interest income Recognized, With no related allowance recorded	16	26	48
Recorded investment, With an allowance recorded	2,048	1,823	3,334
Unpaid principal, With an allowance recorded	2,123	1,869	3,385
Related allowance, With an allowance recorded	1,270	822	1,130
Average recorded Investment, With an allowance recorded	2,691	1,959	2,714
Interest income Recognized, With an allowance recorded	53	55	77
Consumer and Industrial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded Investment, With no related allowance recorded		1	1
Recorded investment, With an allowance recorded	6
Unpaid principal, With an allowance recorded	6
Related allowance, With an allowance recorded	6
Average recorded Investment, With an allowance recorded	3
Construction [Member] | Residential [Member] | Originated Loans [Member]
Financing Receivable, Impaired [Line Items]
Recorded investment, With no related allowance recorded	144	11	11
Unpaid principal, With no related allowance recorded	144	16	16
Average recorded Investment, With no related allowance recorded	78	7	7
Interest income Recognized, With no related allowance recorded	$ 7
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Financing Receivable, Impaired [Line Items]
Average recorded Investment, With no related allowance recorded		$ 245	$ 245