Recorded Investment in Nonaccrual and Loans Past Due Over 90 Days Still On Accrual by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	$ 12,144	$ 17,857
Loans Past Due 90 Days or More & Still Accruing	0	0
Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	2,596	3,471
Loans Past Due 90 Days or More & Still Accruing	0	0
Commercial Real Estate Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	8,847	13,289
Loans Past Due 90 Days or More & Still Accruing	0	0
Agricultural Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	0	0
Loans Past Due 90 Days or More & Still Accruing	0	0
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	111	90
Loans Past Due 90 Days or More & Still Accruing	0	0
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	168	259
Loans Past Due 90 Days or More & Still Accruing	0	0
Residential Mortgage Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-Accrual	422	748
Loans Past Due 90 Days or More & Still Accruing	$ 0	$ 0