Activity in Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	$ 16,003	$ 15,957	$ 16,815	$ 16,372	$ 16,372
Provision charged to expense	1,475	3,200	4,675	9,100	13,100
Losses charged off	2,257	3,037	7,040	9,873	13,818
Recoveries	315	361	1,086	882	1,161
Allowance for loan losses, end of period	15,536	16,481	15,536	16,481	16,815
Allowance for loan losses, individually evaluated for impairment, ending balance	769		769		2,225
Allowance for loan losses, collectively evaluated for impairment, ending balance	14,767		14,767		14,590
Loans, individually evaluated for impairment, ending balance	18,788		18,788		31,324
Loans, collectively evaluated for impairment, ending balance	946,059		946,059		888,885
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	10,004	9,790	10,602	10,124	10,124
Provision charged to expense	985	2,915	2,410	6,145	8,592
Losses charged off	1,484	2,017	4,034	5,582	8,260
Recoveries	16	64	543	65	146
Allowance for loan losses, end of period	9,521	10,752	9,521	10,752	10,602
Allowance for loan losses, individually evaluated for impairment, ending balance	732		732		2,189
Allowance for loan losses, collectively evaluated for impairment, ending balance	8,789		8,789		8,413
Loans, individually evaluated for impairment, ending balance	14,299		14,299		25,409
Loans, collectively evaluated for impairment, ending balance	259,760		259,760		257,440
Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	3,392	2,135	3,444	2,212	2,212
Provision charged to expense	480	100	1,571	2,100	4,390
Losses charged off	505	464	1,652	2,644	3,432
Recoveries	196	63	200	166	274
Allowance for loan losses, end of period	3,563	1,834	3,563	1,834	3,444
Allowance for loan losses, individually evaluated for impairment, ending balance	37		37		36
Allowance for loan losses, collectively evaluated for impairment, ending balance	3,526		3,526		3,408
Loans, individually evaluated for impairment, ending balance	4,489		4,489		5,915
Loans, collectively evaluated for impairment, ending balance	486,523		486,523		429,061
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, beginning of period	2,607	4,032	2,769	4,036	4,036
Provision charged to expense	10	185	694	855	118
Losses charged off	268	556	1,354	1,647	2,126
Recoveries	103	234	343	651	741
Allowance for loan losses, end of period	2,452	3,895	2,452	3,895	2,769
Allowance for loan losses, collectively evaluated for impairment, ending balance	2,452		2,452		2,769
Loans, collectively evaluated for impairment, ending balance	$ 199,776		$ 199,776		$ 202,384