Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	$ 1,043		$ 745
Allowance for loan losses, Collectively evaluated for impairment	13,295		13,958
Allowance for loan losses, Modified in a troubled debt restructuring	2,898		2,670
Acquired with deteriorated credit quality	17,236	$ 17,504	17,373	$ 17,843	$ 17,415	$ 16,234
Loans:
Loans individually evaluated for impairment	4,942		4,545
Loans collectively evaluated for impairment	1,500,941		1,335,674
Acquired with deteriorated credit quality	872,768		781,210
Loans modified in a troubled debt restructuring	13,941		18,658
Total ending loans balance	1,520,519		1,359,596
Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	695		719
Commercial, Industrial, and Agricultural [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	371		254
Allowance for loan losses, Collectively evaluated for impairment	5,288		6,703
Allowance for loan losses, Modified in a troubled debt restructuring	960		157
Acquired with deteriorated credit quality	6,619	6,598	7,114	8,726	8,096	8,212
Loans:
Loans individually evaluated for impairment	4,056		3,394
Loans collectively evaluated for impairment	443,035		421,144
Acquired with deteriorated credit quality	450,969		428,458
Loans modified in a troubled debt restructuring	3,878		3,920
Total ending loans balance	450,969		428,458
Commercial, Industrial, and Agricultural [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Commercial Mortgages [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	486		294
Allowance for loan losses, Collectively evaluated for impairment	3,121		2,503
Allowance for loan losses, Modified in a troubled debt restructuring	1,938		2,513
Acquired with deteriorated credit quality	5,545	5,928	5,310	4,198	4,581	3,536
Loans:
Loans individually evaluated for impairment	486		494
Loans collectively evaluated for impairment	410,555		336,801
Acquired with deteriorated credit quality	421,799		352,752
Loans modified in a troubled debt restructuring	10,063		14,738
Total ending loans balance	421,799		352,752
Commercial Mortgages [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	695		719
Residential Real Estate [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	186		197
Allowance for loan losses, Collectively evaluated for impairment	2,354		2,282
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Acquired with deteriorated credit quality	2,540	2,612	2,479	2,561	2,467	2,450
Loans:
Loans individually evaluated for impairment	400		657
Loans collectively evaluated for impairment	565,645		501,660
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	566,045		502,317
Residential Real Estate [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Consumer [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	2,276		2,205
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Acquired with deteriorated credit quality	2,276	2,118	2,205	2,073	1,996	1,763
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	75,126		69,648
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	75,126		69,648
Consumer [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Credit Cards [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	98		71
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Acquired with deteriorated credit quality	98	98	71	79	88	66
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	5,037		5,233
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	5,037		5,233
Credit Cards [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Overdrafts [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0		0
Allowance for loan losses, Collectively evaluated for impairment	158		194
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Acquired with deteriorated credit quality	158	$ 150	194	$ 206	$ 187	$ 207
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	1,543		1,188
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	1,543		1,188
Overdrafts [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Ending allowance balance attributable to loans:
Acquired with deteriorated credit quality	0		0
Loans:
Acquired with deteriorated credit quality	$ 0		$ 0