Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 58,656	$ 54,999	$ 55,686	$ 54,324
Charge-offs	(3,164)	(2,766)	(5,388)	(6,484)
Recoveries	1,001	929	1,932	1,681
Provision	4,831	889	9,094	4,530
Ending Balance	61,324	54,051	61,324	54,051
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,716	3,983	4,258	4,210
Charge-offs	(212)	(17)	(212)	(888)
Recoveries	0	2	14	3
Provision	1,205	(136)	1,649	507
Ending Balance	5,709	3,832	5,709	3,832
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,141	2,183	2,224	2,263
Charge-offs	(195)	(134)	(211)	(399)
Recoveries	1	5	76	7
Provision	(90)	209	(232)	392
Ending Balance	1,857	2,263	1,857	2,263
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	8,935	8,856	9,156	8,767
Charge-offs	(1,342)	(1,488)	(2,631)	(3,232)
Recoveries	377	266	667	569
Provision	1,352	1,293	2,130	2,823
Ending Balance	9,322	8,927	9,322	8,927
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	19,395	16,180	18,098	14,765
Charge-offs	(978)	(1,126)	(1,772)	(1,356)
Recoveries	300	54	404	288
Provision	1,992	2,060	3,979	3,471
Ending Balance	20,709	17,168	20,709	17,168
Commercial real estate | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	23,469	23,797	21,950	24,319
Charge-offs	(437)	(1)	(562)	(609)
Recoveries	323	602	771	814
Provision	372	(2,537)	1,568	(2,663)
Ending Balance	$ 23,727	$ 21,861	$ 23,727	$ 21,861