Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	$ 1,505		$ 753
Allowance for loan losses, Collectively evaluated for impairment	13,816		12,208
Allowance for loan losses, Modified in a troubled debt restructuring	1,900		1,099
Total ending allowance balance	17,221	15,500	14,060	13,649	13,690	12,615
Loans:
Loans individually evaluated for impairment	6,090		13,899
Loans collectively evaluated for impairment	1,009,626		902,304
Loans modified in a troubled debt restructuring	13,255		11,621
Total ending loans balance	1,028,971		927,824
Commercial, Industrial, and Agricultural [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	573		541
Allowance for loan losses, Collectively evaluated for impairment	4,980		4,399
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Total ending allowance balance	5,553	5,338	4,940	4,473	5,115	4,511
Loans:
Loans individually evaluated for impairment	1,774		2,623
Loans collectively evaluated for impairment	255,813		253,048
Loans modified in a troubled debt restructuring	1,302		1,420
Total ending loans balance	258,889		257,091
Commercial Mortgages [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	932		131
Allowance for loan losses, Collectively evaluated for impairment	4,209		3,467
Allowance for loan losses, Modified in a troubled debt restructuring	1,900		1,099
Total ending allowance balance	7,041	5,896	4,697	4,892	4,553	4,470
Loans:
Loans individually evaluated for impairment	4,252		10,683
Loans collectively evaluated for impairment	289,635		240,907
Loans modified in a troubled debt restructuring	11,953		10,201
Total ending loans balance	305,840		261,791
Residential Real Estate [Member]
Ending allowance balance attributable to loans:
Allowance for loan losses, Individually evaluated for impairment	0		81
Allowance for loan losses, Collectively evaluated for impairment	2,641		2,385
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Total ending allowance balance	2,641	2,508	2,466	2,332	2,225	1,991
Loans:
Loans individually evaluated for impairment	64		593
Loans collectively evaluated for impairment	400,585		347,311
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	400,649		347,904
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	1,761		1,699
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Total ending allowance balance	1,761	1,555	1,699	1,706	1,568	1,404
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	58,283		55,267
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	58,283		55,267
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	57		83
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Total ending allowance balance	57	55	83	79	81	71
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	4,800		4,800
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	4,800		4,800
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	168		175
Allowance for loan losses, Modified in a troubled debt restructuring	0		0
Total ending allowance balance	168	148	175	167	148	168
Loans:
Loans individually evaluated for impairment	0		0
Loans collectively evaluated for impairment	510		971
Loans modified in a troubled debt restructuring	0		0
Total ending loans balance	$ 510		$ 971