Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 2,902,209	$ 2,713,415
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	622,579	532,523
Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	112,527	104,526
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	98,639	81,780
Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,266,682	1,194,230
Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	473,537	481,493
Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	203,406	191,631
Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	75,748	84,172
Finance Leases
Financing Receivable, Recorded Investment [Line Items]
Loans	2,590	3,555
Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	46,501	39,505
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	2,107,317	1,855,248
Commercial Pass | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	559,852	478,885
Commercial Pass | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	112,209	101,289
Commercial Pass | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	85,728	47,611
Commercial Pass | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	1,148,561	1,033,397
Commercial Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	145,402	139,237
Commercial Pass | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	9,092	15,912
Commercial Pass | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	46,473	38,917
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	69,815	92,045
Commercial Special Mention | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	23,678	22,405
Commercial Special Mention | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	224	1,582
Commercial Special Mention | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	1,384	3,672
Commercial Special Mention | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	38,199	54,697
Commercial Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,437	9,175
Commercial Special Mention | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	893	499
Commercial Special Mention | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		15
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	140,553	175,423
Commercial Substandard | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	34,460	28,025
Commercial Substandard | Agriculture production financing and other loans
Financing Receivable, Recorded Investment [Line Items]
Loans	94	1,655
Commercial Substandard | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	11,356	22,376
Commercial Substandard | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	79,078	103,330
Commercial Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	13,880	16,699
Commercial Substandard | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,657	3,317
Commercial Substandard | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	28	21
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	6,064	5,984
Commercial Doubtful | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans	4,589	3,208
Commercial Doubtful | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	553	1,724
Commercial Doubtful | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	922	500
Commercial Doubtful | Other Loans
Financing Receivable, Recorded Investment [Line Items]
Loans		552
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	569,372	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	301,614	308,306
Consumer Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	189,721	170,776
Consumer Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	75,748	84,121
Consumer Performing | Finance Leases
Financing Receivable, Recorded Investment [Line Items]
Loans	2,289	3,555
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	9,088	9,160
Consumer Non-Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	171	359
Consumer Non-Performing | Commercial and farm land
Financing Receivable, Recorded Investment [Line Items]
Loans	291	47
Consumer Non-Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	6,282	7,576
Consumer Non-Performing | Home Equity
Financing Receivable, Recorded Investment [Line Items]
Loans	2,043	1,127
Consumer Non-Performing | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans		51
Consumer Non-Performing | Finance Leases
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 301