Loans Individually Evaluated for Impairment by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 12,051		$ 12,051		$ 16,945
Recorded Investment	11,954		11,954		16,691
Allowance for Loan Losses Allocated	5,530		5,530		4,911
Average Recorded Investment	13,420	15,711	15,168	18,028
Interest Income Recognized	23	37	46	120
Cash Basis Recognized	21	36	42	117
Impaired Financing Receivables with No Related Allowance [Member] | Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	116		116		1,084
Recorded Investment	95		95		1,066
Allowance for Loan Losses Allocated	0		0		0
Average Recorded Investment	139	105	298	345
Interest Income Recognized	1	6	3	9
Cash Basis Recognized	1	6	3	9
Impaired Financing Receivables with No Related Allowance [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,116		2,116		5,959
Recorded Investment	2,066		2,066		5,894
Allowance for Loan Losses Allocated	0		0		0
Average Recorded Investment	3,353	4,362	5,023	3,603
Interest Income Recognized	12	14	17	46
Cash Basis Recognized	12	14	17	46
Impaired Financing Receivables with No Related Allowance [Member] | Agricultural Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	0		0		0
Recorded Investment	0		0		0
Allowance for Loan Losses Allocated	0		0		0
Average Recorded Investment	0	0	49	25
Interest Income Recognized	0	0	2	6
Cash Basis Recognized	0	0	2	6
Impaired Financing Receivables with Related Allowance [Member] | Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,696		2,696		2,502
Recorded Investment	2,683		2,683		2,501
Allowance for Loan Losses Allocated	1,357		1,357		466
Average Recorded Investment	2,749	3,467	2,795	4,009
Interest Income Recognized	4	3	7	9
Cash Basis Recognized	3	3	6	9
Impaired Financing Receivables with Related Allowance [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	7,123		7,123		7,400
Recorded Investment	7,110		7,110		7,230
Allowance for Loan Losses Allocated	4,173		4,173		4,445
Average Recorded Investment	7,179	7,777	7,003	10,046
Interest Income Recognized	6	14	17	50
Cash Basis Recognized	5	13	14	47
Impaired Financing Receivables with Related Allowance [Member] | Agricultural Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	0		0		0
Recorded Investment	0		0		0
Allowance for Loan Losses Allocated	0		0		0
Average Recorded Investment	0	0	0	0
Interest Income Recognized	0	0	0	0
Cash Basis Recognized	$ 0	$ 0	$ 0	$ 0