Loans and Allowance - Composition of Loan Portfolio by Loan Class (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Loans:
Loans	$ 3,722,733		$ 3,632,409
Allowance for loan losses	(68,367)	(69,583)	(67,870)	(68,202)	(68,537)	(69,366)
Net Loans	3,654,366		3,564,539
Commercial and industrial loans
Loans:
Loans	857,844		761,705
Agricultural production financing and other loans to farmers
Loans:
Loans	102,270		114,348
Real estate loans - Construction
Loans:
Loans	165,388		177,082
Real estate loans - Commercial and farm land
Loans:
Loans	1,621,436		1,611,809
Real estate loans - Residential
Loans:
Loans	629,162		616,385
Allowance for loan losses	(14,928)	(13,908)	(15,077)	(15,113)	(15,479)	(14,157)
Real estate loans - Home equity
Loans:
Loans	261,811		255,223
Individuals' loans for household and other personal expenditures
Loans:
Loans	61,533		69,783
Allowance for loan losses	(2,243)	(2,410)	(2,515)	(2,885)	(2,689)	(2,593)
Lease financing receivables, net of unearned income
Loans:
Loans	1,231		1,545
Allowance for loan losses				(36)	(20)
Other loans
Loans:
Loans	$ 22,058		$ 24,529