Loans Individually Evaluated for Impairment by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	$ 15,052	[1]	$ 18,811	[1]
Recorded Investment	12,663		16,691
Allowance for Loan Losses Allocated	5,411		4,911
Average Recorded Investment	14,753		18,596		10,166
Interest Income Recognized	55		138		78
Cash Basis Recognized	50		135		78
Impaired Financing Receivables with No Related Allowance [Member] | Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	108	[1]	1,731	[1]
Recorded Investment	87		1,066
Allowance for Loan Losses Allocated	0		0
Average Recorded Investment	252		1,107
Interest Income Recognized	3		9
Cash Basis Recognized	3		9
Impaired Financing Receivables with No Related Allowance [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	4,312	[1]	6,991	[1]
Recorded Investment	2,154		5,894
Allowance for Loan Losses Allocated	0		0
Average Recorded Investment	4,506		4,438
Interest Income Recognized	18		75
Cash Basis Recognized	18		75
Impaired Financing Receivables with No Related Allowance [Member] | Agricultural Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,126	[1]	0	[1]
Recorded Investment	2,137		0
Allowance for Loan Losses Allocated	0		0
Average Recorded Investment	535		19
Interest Income Recognized	2		6
Cash Basis Recognized	2		6
Impaired Financing Receivables with No Related Allowance [Member] | Subtotal [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	6,546	[1]	8,722	[1]
Recorded Investment	4,378		6,960
Allowance for Loan Losses Allocated	0		0
Average Recorded Investment	5,293		5,564
Interest Income Recognized	23		90
Cash Basis Recognized	23		90
Impaired Financing Receivables with Related Allowance [Member] | Commercial and Industrial Loans and Leases [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	2,642	[1]	2,502	[1]
Recorded Investment	2,581		2,501
Allowance for Loan Losses Allocated	1,347		466
Average Recorded Investment	2,726		3,642
Interest Income Recognized	9		11
Cash Basis Recognized	8		11
Impaired Financing Receivables with Related Allowance [Member] | Commercial Real Estate Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	5,579	[1]	7,587	[1]
Recorded Investment	5,418		7,230
Allowance for Loan Losses Allocated	3,914		4,445
Average Recorded Investment	6,660		9,390
Interest Income Recognized	23		37
Cash Basis Recognized	19		34
Impaired Financing Receivables with Related Allowance [Member] | Agricultural Loans [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	285	[1]	0	[1]
Recorded Investment	286		0
Allowance for Loan Losses Allocated	150		0
Average Recorded Investment	74		0
Interest Income Recognized	0		0
Cash Basis Recognized	0		0
Impaired Financing Receivables with Related Allowance [Member] | Subtotal [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	8,506	[1]	10,089	[1]
Recorded Investment	8,285		9,731
Allowance for Loan Losses Allocated	5,411		4,911
Average Recorded Investment	9,460		13,032
Interest Income Recognized	32		48
Cash Basis Recognized	27		45
Loans Acquired With Deteriorated Credit Quality With No Related Allowance Recorded [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	45	[1]	48	[1]
Recorded Investment	25		28
Allowance for Loan Losses Allocated	0		0
Average Recorded Investment	26		28
Interest Income Recognized	2		4
Cash Basis Recognized	2		4
Loans Acquired With Deteriorated Credit Quality With Additional Allowance Recorded [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance	155	[1]	205	[1]
Recorded Investment	118		77
Allowance for Loan Losses Allocated	88		77
Average Recorded Investment	154		77
Interest Income Recognized	6		1
Cash Basis Recognized	$ 4		$ 1

[1] Unpaid Principal Balance is the remaining contractual payments inclusive of partial charge-offs.