Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 2,879,542	$ 2,902,209
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	637,952	622,579
Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	104,284	112,527
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	75,877	98,639
Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,280,611	1,266,682
Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	467,629	473,537
Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	201,767	203,406
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	73,314	75,748
Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	2,013	2,590
Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	36,095	46,501
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	2,109,099	2,107,317
Commercial Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	588,140	559,852
Commercial Pass | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	103,894	112,209
Commercial Pass | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	64,904	85,728
Commercial Pass | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,157,933	1,148,561
Commercial Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	145,949	145,402
Commercial Pass | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	10,334	9,092
Commercial Pass | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	1,866
Commercial Pass | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	36,079	46,473
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	71,550	69,815
Commercial Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	17,592	23,678
Commercial Special Mention | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	217	224
Commercial Special Mention | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	1,372	1,384
Commercial Special Mention | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	48,235	38,199
Commercial Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	3,480	5,437
Commercial Special Mention | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	638	893
Commercial Special Mention | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	16
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	129,102	140,553
Commercial Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	30,027	34,460
Commercial Substandard | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	173	94
Commercial Substandard | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	9,091	11,356
Commercial Substandard | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	73,845	79,078
Commercial Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	14,579	13,880
Commercial Substandard | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,240	1,657
Commercial Substandard | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	147
Commercial Substandard | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		28
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	2,687	6,064
Commercial Doubtful | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	2,193	4,589
Commercial Doubtful | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	350	553
Commercial Doubtful | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	144	922
Commercial Doubtful | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	557,793	569,372
Consumer Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	296,613	301,614
Consumer Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	187,877	189,721
Consumer Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	73,303	75,748
Consumer Performing | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans		2,289
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	9,311	9,088
Consumer Non-Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	510	171
Consumer Non-Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	248	291
Consumer Non-Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	6,864	6,282
Consumer Non-Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,678	2,043
Consumer Non-Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	11
Consumer Non-Performing | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 301