Loans - Aging of Recorded Investment in Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 13,857	$ 14,455
Loans Not Past Due	2,088,338	1,862,511
Total	2,102,195	1,876,966
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,062	5,025
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,799	1,926
Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,996	7,504
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,260	3,151
Loans Not Past Due	660,684	564,649
Total	663,944	567,800
Commercial, Industrial, and Agricultural [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,827	1,558
Commercial, Industrial, and Agricultural [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	486	299
Commercial, Industrial, and Agricultural [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	947	1,294
Commercial Mortgages [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	744	2,075
Loans Not Past Due	655,386	572,751
Total	656,130	574,826
Commercial Mortgages [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	559
Commercial Mortgages [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	40	0
Commercial Mortgages [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	704	1,516
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,267	6,602
Loans Not Past Due	688,792	646,281
Total	697,059	652,883
Residential Real Estate [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,688	2,155
Residential Real Estate [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,932	737
Residential Real Estate [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,647	3,710
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,466	2,512
Loans Not Past Due	76,798	72,304
Total	78,264	74,816
Consumer [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	484	648
Consumer [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	305	890
Consumer [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	677	974
Credit Cards [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	120	115
Loans Not Past Due	6,129	5,931
Total	6,249	6,046
Credit Cards [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	63	105
Credit Cards [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	36	0
Credit Cards [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	21	10
Overdrafts [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Loans Not Past Due	549	595
Total	549	595
Overdrafts [Member] | 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Overdrafts [Member] | 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Overdrafts [Member] | Greater than 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0	$ 0