Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 2,797,634	$ 2,713,415
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	552,353	532,523
Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	106,135	104,526
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	99,588	81,780
Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,219,114	1,194,230
Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	480,917	481,493
Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	207,250	191,631
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	83,933	84,172
Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	2,976	3,555
Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	45,368	39,505
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	1,938,595	1,855,248
Commercial Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	485,220	478,885
Commercial Pass | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	105,269	101,289
Commercial Pass | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	76,112	47,611
Commercial Pass | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,079,776	1,033,397
Commercial Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	132,361	139,237
Commercial Pass | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	14,543	15,912
Commercial Pass | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	45,314	38,917
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	95,792	92,045
Commercial Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	37,086	22,405
Commercial Special Mention | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	352	1,582
Commercial Special Mention | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	2,116	3,672
Commercial Special Mention | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	45,377	54,697
Commercial Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	9,552	9,175
Commercial Special Mention | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,298	499
Commercial Special Mention | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	11	15
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	158,860	175,423
Commercial Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	25,293	28,025
Commercial Substandard | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	514	1,655
Commercial Substandard | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	21,246	22,376
Commercial Substandard | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	93,101	103,330
Commercial Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	16,885	16,699
Commercial Substandard | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,778	3,317
Commercial Substandard | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	43	21
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	6,015	5,984
Commercial Doubtful | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	4,754	3,208
Commercial Doubtful | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	802	1,724
Commercial Doubtful | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	459	500
Commercial Doubtful | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		552
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	589,200	575,555
Consumer Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans		7,762
Consumer Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans		1,035
Consumer Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	314,191	308,306
Consumer Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	188,108	170,776
Consumer Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	83,925	84,121
Consumer Performing | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	2,976	3,555
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	9,172	9,160
Consumer Non-Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	114	359
Consumer Non-Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	58	47
Consumer Non-Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	7,469	7,576
Consumer Non-Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,523	1,127
Consumer Non-Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 8	$ 51