Commercial Non-Impaired Loans Included in ASC 450 Loss Migration Analysis (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Loan	Sep. 30, 2011 Loan	Sep. 30, 2012 Loan	Sep. 30, 2011 Loan
Financing Receivable, Impaired [Line Items]
Recorded Balance	$ 2,505		$ 6,695
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Recorded Balance	1,048		1,302
Commercial and farm land
Financing Receivable, Impaired [Line Items]
Recorded Balance	875		2,758
Residential
Financing Receivable, Impaired [Line Items]
Number of Loans	3		11
Recorded Balance	167		1,880
Nonimpaired Loans
Financing Receivable, Impaired [Line Items]
Number of Loans	11	5	15	7
Recorded Balance	1,809	348	3,932	1,221
Nonimpaired Loans | Commercial and industrial
Financing Receivable, Impaired [Line Items]
Number of Loans	3	1	3	2
Recorded Balance	1,415	66	1,415	553
Nonimpaired Loans | Commercial and farm land
Financing Receivable, Impaired [Line Items]
Number of Loans	1		1	1
Recorded Balance	23		23
Nonimpaired Loans | Residential
Financing Receivable, Impaired [Line Items]
Number of Loans	7	3	11	3
Recorded Balance	371	271	2,494	657
Nonimpaired Loans | Home Equity
Financing Receivable, Impaired [Line Items]
Number of Loans		1		1
Recorded Balance		$ 11		$ 11