Fair Value (Assets and Liabilities Measured at Fair Value on Non-Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial and Industrial Loans and Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	$ 0	$ 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial Real Estate Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Impaired Loans with Specific Allocations [Member] | Agricultural Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Real Estate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial and Industrial Loans and Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial Real Estate Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Impaired Loans with Specific Allocations [Member] | Agricultural Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Other Real Estate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	0
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans with Specific Allocations [Member] | Commercial and Industrial Loans and Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	1,406	1,231
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans with Specific Allocations [Member] | Commercial Real Estate Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	1,753	1,497
Fair Value, Measurements, Nonrecurring [Member] | Impaired Loans with Specific Allocations [Member] | Agricultural Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	135
Fair Value, Measurements, Nonrecurring [Member] | Other Real Estate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	376	150
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial and Industrial Loans and Leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	1,406	1,231
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans with Specific Allocations [Member] | Commercial Real Estate Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	1,753	1,497
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Impaired Loans with Specific Allocations [Member] | Agricultural Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	0	135
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other Real Estate [Member] | Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Assets	$ 376	$ 150