Activity in Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	$ 16,815	$ 16,372	$ 16,414
Provision charged to expense	6,025	13,100	7,050
Losses charged off	(8,002)	(13,818)	(8,292)
Recoveries	1,200	1,161	1,200
Allowance for loan losses, end of period	16,038	16,815	16,372
Allowance for loan losses, individually evaluated for impairment, ending balance	1,373	2,225	1,409
Allowance for loan losses, collectively evaluated for impairment, ending balance	14,665	14,590	14,963
Loans, individually evaluated for impairment, ending balance	19,121	31,324	27,237
Loans, collectively evaluated for impairment, ending balance	971,448	888,885	972,498
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	10,602	10,124	10,003
Provision charged to expense	3,213	8,592	1,780
Losses charged off	(4,493)	(8,260)	(1,752)
Recoveries	586	146	93
Allowance for loan losses, end of period	9,908	10,602	10,124
Allowance for loan losses, individually evaluated for impairment, ending balance	1,316	2,189	1,340
Allowance for loan losses, collectively evaluated for impairment, ending balance	8,592	8,413	8,784
Loans, individually evaluated for impairment, ending balance	15,704	25,409	20,309
Loans, collectively evaluated for impairment, ending balance	273,144	257,440	293,622
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	3,444	2,212	2,359
Provision charged to expense	1,612	4,390	2,900
Losses charged off	(1,901)	(3,432)	(3,345)
Recoveries	239	274	298
Allowance for loan losses, end of period	3,394	3,444	2,212
Allowance for loan losses, individually evaluated for impairment, ending balance	57	36	69
Allowance for loan losses, collectively evaluated for impairment, ending balance	3,337	3,408	2,143
Loans, individually evaluated for impairment, ending balance	3,417	5,915	6,928
Loans, collectively evaluated for impairment, ending balance	499,202	429,061	451,091
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	2,769	4,036	4,052
Provision charged to expense	1,200	118	2,370
Losses charged off	(1,608)	(2,126)	(3,195)
Recoveries	375	741	809
Allowance for loan losses, end of period	2,736	2,769	4,036
Allowance for loan losses, collectively evaluated for impairment, ending balance	2,736	2,769	4,036
Loans, collectively evaluated for impairment, ending balance	$ 199,102	$ 202,384	$ 227,785