Composition of Loan Portfolio by Loan Class (Detail) (USD $)	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Loans:
Loans	$ 2,797,634,000		$ 2,713,415,000
Allowance for loan losses	(70,143,000)	(70,369,000)	(70,898,000)	(77,133,000)	(80,936,000)	(82,977,000)
Total Loans	2,727,491,000		2,642,517,000
Commercial and industrial
Loans:
Loans	552,353,000		532,523,000
Agriculture production financing and other loans
Loans:
Loans	106,135,000		104,526,000
Construction
Loans:
Loans	99,588,000		81,780,000
Commercial and farm land
Loans:
Loans	1,219,114,000		1,194,230,000
Residential
Loans:
Loans	480,917,000		481,493,000
Allowance for loan losses	(14,788,000)	(14,083,000)	(12,343,000)	(11,877,000)	(10,371,000)	(10,408,000)
Home Equity
Loans:
Loans	207,250,000		191,631,000
Consumer
Loans:
Loans	83,933,000		84,172,000
Allowance for loan losses	(2,431,000)	(2,805,000)	(2,902,000)	(2,859,000)	(3,098,000)	(3,622,000)
Finance Lease
Loans:
Loans	2,976,000		3,555,000
Allowance for loan losses			(3,000)	(14,000)	(21,000)	(98,000)
Other Loans
Loans:
Loans	$ 45,368,000		$ 39,505,000