Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 2,836,324	$ 2,713,415
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	592,517	532,523
Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	107,166	104,526
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	93,610	81,780
Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,241,054	1,194,230
Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	475,272	481,493
Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	204,888	191,631
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	77,171	84,172
Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	2,970	3,555
Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	41,676	39,505
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	2,010,087	1,855,248
Commercial Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	524,180	478,885
Commercial Pass | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	106,775	101,289
Commercial Pass | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	78,655	47,611
Commercial Pass | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,105,407	1,033,397
Commercial Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	142,394	139,237
Commercial Pass | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	11,036	15,912
Commercial Pass | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	41,640	38,917
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	81,609	92,045
Commercial Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	25,143	22,405
Commercial Special Mention | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	230	1,582
Commercial Special Mention | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	1,639	3,672
Commercial Special Mention | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	48,344	54,697
Commercial Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	4,909	9,175
Commercial Special Mention | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,344	499
Commercial Special Mention | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		15
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	152,131	175,423
Commercial Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	36,564	28,025
Commercial Substandard | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	161	1,655
Commercial Substandard | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	13,140	22,376
Commercial Substandard | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	86,500	103,330
Commercial Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	14,709	16,699
Commercial Substandard | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,021	3,317
Commercial Substandard | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	36	21
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	8,465	5,984
Commercial Doubtful | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	6,630	3,208
Commercial Doubtful | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	475	1,724
Commercial Doubtful | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	1,360	500
Commercial Doubtful | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		552
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	573,994	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	304,255	308,306
Consumer Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	189,938	170,776
Consumer Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	77,168	84,121
Consumer Performing | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	2,633	3,555
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	10,038	9,160
Consumer Non-Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	176	359
Consumer Non-Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	328	47
Consumer Non-Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	7,645	7,576
Consumer Non-Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,549	1,127
Consumer Non-Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	3	51
Consumer Non-Performing | Finance Lease
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 337