Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 55,686	$ 54,324
Charge-offs	(2,224)	(3,718)
Recoveries	931	752
Provision	4,263	3,641
Ending Balance	58,656	54,999
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,258	4,210
Charge-offs	0	(871)
Recoveries	14	1
Provision	444	643
Ending Balance	4,716	3,983
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,224	2,263
Charge-offs	(16)	(265)
Recoveries	75	2
Provision	(142)	183
Ending Balance	2,141	2,183
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,156	8,767
Charge-offs	(1,289)	(1,744)
Recoveries	290	303
Provision	778	1,530
Ending Balance	8,935	8,856
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	18,098	14,765
Charge-offs	(794)	(230)
Recoveries	104	234
Provision	1,987	1,411
Ending Balance	19,395	16,180
Commercial real estate | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	21,950	24,319
Charge-offs	(125)	(608)
Recoveries	448	212
Provision	1,196	(126)
Ending Balance	$ 23,469	$ 23,797