Loans and Allowance - Composition of Loan Portfolio by Loan Class (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Loans:
Loans	$ 3,772,467		$ 3,632,409
Allowance for loan losses	(65,596)	(68,367)	(67,870)	(66,224)	(68,202)	(69,366)
Net Loans	3,706,871		3,564,539
Commercial and industrial loans
Loans:
Loans	900,970		761,705
Agricultural production financing and other loans to farmers
Loans:
Loans	99,649		114,348
Real estate loans - Construction
Loans:
Loans	178,213		177,082
Real estate loans - Commercial and farm land
Loans:
Loans	1,603,698		1,611,809
Real estate loans - Residential
Loans:
Loans	625,609		616,385
Allowance for loan losses	(14,207)	(14,928)	(15,077)	(14,706)	(15,113)	(14,157)
Real estate loans - Home equity
Loans:
Loans	269,952		255,223
Individuals' loans for household and other personal expenditures
Loans:
Loans	66,832		69,783
Allowance for loan losses	(2,228)	(2,243)	(2,515)	(2,803)	(2,885)	(2,593)
Lease financing receivables, net of unearned income
Loans:
Loans	1,208		1,545
Allowance for loan losses	(1)			(56)	(36)
Other loans
Loans:
Loans	$ 26,336		$ 24,529