Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	$ 5,376	$ 3,626
60-89 Days Past Due	2,859	2,489
Greater than 90 Days	17,303	25,200
Total Past Due	25,538	31,315
Current	857,010	811,773
Total ending loans balance	882,548	843,088	812,579
Recorded Investment greater than 90 Days and Accruing	184	0
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	3,681	290
60-89 Days Past Due	1,004	804
Greater than 90 Days	12,398	19,023
Total Past Due	17,083	20,117
Current	396,922	361,735
Total ending loans balance	414,005	381,852	375,803
Recorded Investment greater than 90 Days and Accruing	0	0
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	54
60-89 Days Past Due	0	249
Greater than 90 Days	376	805
Total Past Due	376	1,108
Current	68,329	75,462
Total ending loans balance	68,705	76,570	65,662
Recorded Investment greater than 90 Days and Accruing	0	0
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	394	545
60-89 Days Past Due	1,094	1,172
Greater than 90 Days	2,827	3,554
Total Past Due	4,315	5,271
Current	60,668	59,253
Total ending loans balance	64,983	64,524	74,685
Recorded Investment greater than 90 Days and Accruing	184	0
Home equity loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	630	1,942
60-89 Days Past Due	494	181
Greater than 90 Days	1,510	1,666
Total Past Due	2,634	3,789
Current	120,196	123,169
Total ending loans balance	122,830	126,958	132,536
Recorded Investment greater than 90 Days and Accruing	0	0
Indirect
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	645	664
60-89 Days Past Due	227	71
Greater than 90 Days	69	124
Total Past Due	941	859
Current	198,983	179,230
Total ending loans balance	199,924	180,089	150,031
Recorded Investment greater than 90 Days and Accruing
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	26	131
60-89 Days Past Due	40	12
Greater than 90 Days	123	28
Total Past Due	189	171
Current	11,912	12,924
Total ending loans balance	12,101	13,095	13,862
Recorded Investment greater than 90 Days and Accruing	$ 0	$ 0