Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 61,324	$ 54,051	$ 55,686	$ 54,324
Charge-offs	(6,120)	(5,759)	(11,508)	(12,243)
Recoveries	779	888	2,711	2,569
Provision	5,238	5,705	14,332	10,235
Ending Balance	61,221	54,885	61,221	54,885
Agricultural and agricultural real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	5,709	3,832	4,258	4,210
Charge-offs	(1,145)	0	(1,357)	(888)
Recoveries	0	14	14	17
Provision	2	2,281	1,651	2,788
Ending Balance	4,566	6,127	4,566	6,127
Residential real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,857	2,263	2,224	2,263
Charge-offs	0	(142)	(211)	(541)
Recoveries	1	63	77	70
Provision	(8)	82	(240)	474
Ending Balance	1,850	2,266	1,850	2,266
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,322	8,927	9,156	8,767
Charge-offs	(1,831)	(1,750)	(4,462)	(4,982)
Recoveries	378	418	1,045	987
Provision	1,177	1,387	3,307	4,210
Ending Balance	9,046	8,982	9,046	8,982
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	20,709	17,168	18,098	14,765
Charge-offs	(2,945)	(1,954)	(4,717)	(3,310)
Recoveries	158	347	562	635
Provision	4,147	1,409	8,126	4,880
Ending Balance	22,069	16,970	22,069	16,970
Commercial real estate | Commercial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	23,727	21,861	21,950	24,319
Charge-offs	(199)	(1,913)	(761)	(2,522)
Recoveries	242	46	1,013	860
Provision	(80)	546	1,488	(2,117)
Ending Balance	$ 23,690	$ 20,540	$ 23,690	$ 20,540