Allowance for Credit Losses, Allocation of The Allowance For Loan Losses by Portfolio Segment and By Impairment Methodology (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses [Roll Forward]
Beginning Balance				$ 47,919				$ 41,523	$ 47,919	$ 41,523
Charge-Offs									(685)	(172)
Recoveries									258	233
Provision	$ 0	$ 1,600	$ 650	600	$ 3,485	$ 250	$ 0	2,600	2,850	6,335	$ 750
Ending Balance	50,342				47,919				50,342	47,919	41,523
Ending Balance Individually Evaluated for Impairment	684				831				684	831
Ending Balance Collectively Evaluated for Impairment	49,658				47,088				49,658	47,088
Loans & Leases [Abstract]
Ending Balance	2,215,295				2,177,601				2,215,295	2,177,601
Ending Balance Individually Evaluated for Impairment	9,279				10,863				9,279	10,863
Ending Balance Collectively Evaluated for Impairment	2,206,016				2,166,738				2,206,016	2,166,738
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				11,110				10,063	11,110	10,063
Charge-Offs									(109)	0
Recoveries									109	2
Provision									(188)	1,045
Ending Balance	10,922				11,110				10,922	11,110	10,063
Ending Balance Individually Evaluated for Impairment	366				0				366	0
Ending Balance Collectively Evaluated for Impairment	10,556				11,110				10,556	11,110
Loans & Leases [Abstract]
Ending Balance	684,961				668,046				684,961	668,046
Ending Balance Individually Evaluated for Impairment	4,822				1,932				4,822	1,932
Ending Balance Collectively Evaluated for Impairment	680,139				666,114				680,139	666,114
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				9,450				6,881	9,450	6,881
Charge-Offs									0	0
Recoveries									0	0
Provision									2,635	2,569
Ending Balance	12,085				9,450				12,085	9,450	6,881
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	12,085				9,450				12,085	9,450
Loans & Leases [Abstract]
Ending Balance	499,231				467,685				499,231	467,685
Ending Balance Individually Evaluated for Impairment	0				1,304				0	1,304
Ending Balance Collectively Evaluated for Impairment	499,231				466,381				499,231	466,381
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				3,223				2,485	3,223	2,485
Charge-Offs									0	0
Recoveries									0	0
Provision									(1,377)	738
Ending Balance	1,846				3,223				1,846	3,223	2,485
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	1,846				3,223				1,846	3,223
Loans & Leases [Abstract]
Ending Balance	100,206				176,462				100,206	176,462
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	100,206				176,462				100,206	176,462
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				865				789	865	789
Charge-Offs									(53)	(21)
Recoveries									40	26
Provision									(37)	71
Ending Balance	815				865				815	865	789
Ending Balance Individually Evaluated for Impairment	73				70				73	70
Ending Balance Collectively Evaluated for Impairment	742				795				742	795
Loans & Leases [Abstract]
Ending Balance	260,751				242,247				260,751	242,247
Ending Balance Individually Evaluated for Impairment	2,373				2,126				2,373	2,126
Ending Balance Collectively Evaluated for Impairment	258,378				240,121				258,378	240,121
Home Equity Lines and Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				2,140				2,146	2,140	2,146
Charge-Offs									(3)	(46)
Recoveries									8	103
Provision									179	(63)
Ending Balance	2,324				2,140				2,324	2,140	2,146
Ending Balance Individually Evaluated for Impairment	17				18				17	18
Ending Balance Collectively Evaluated for Impairment	2,307				2,122				2,307	2,122
Loans & Leases [Abstract]
Ending Balance	34,525				31,625				34,525	31,625
Ending Balance Individually Evaluated for Impairment	340				402				340	402
Ending Balance Collectively Evaluated for Impairment	34,185				31,223				34,185	31,223
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				7,381				6,308	7,381	6,308
Charge-Offs									(374)	0
Recoveries									17	0
Provision									1,135	1,073
Ending Balance	8,159				7,381				8,159	7,381	6,308
Ending Balance Individually Evaluated for Impairment	0				128				0	128
Ending Balance Collectively Evaluated for Impairment	8,159				7,253				8,159	7,253
Loans & Leases [Abstract]
Ending Balance	273,582				295,325				273,582	295,325
Ending Balance Individually Evaluated for Impairment	0				625				0	625
Ending Balance Collectively Evaluated for Impairment	273,582				294,700				273,582	294,700
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				8,515				7,836	8,515	7,836
Charge-Offs									0	0
Recoveries									8	47
Provision									674	632
Ending Balance	9,197				8,515				9,197	8,515	7,836
Ending Balance Individually Evaluated for Impairment	220				608				220	608
Ending Balance Collectively Evaluated for Impairment	8,977				7,907				8,977	7,907
Loans & Leases [Abstract]
Ending Balance	265,703				217,577				265,703	217,577
Ending Balance Individually Evaluated for Impairment	1,734				4,464				1,734	4,464
Ending Balance Collectively Evaluated for Impairment	263,969				213,113				263,969	213,113
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				200				175	200	175
Charge-Offs									(146)	(105)
Recoveries									76	55
Provision									79	75
Ending Balance	209				200				209	200	175
Ending Balance Individually Evaluated for Impairment	8				7				8	7
Ending Balance Collectively Evaluated for Impairment	201				193				201	193
Loans & Leases [Abstract]
Ending Balance	6,656				6,913				6,656	6,913
Ending Balance Individually Evaluated for Impairment	10				10				10	10
Ending Balance Collectively Evaluated for Impairment	6,646				6,903				6,646	6,903
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				3,586				3,294	3,586	3,294
Charge-Offs									0	0
Recoveries									0	0
Provision									(223)	292
Ending Balance	3,363				3,586				3,363	3,586	3,294
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	3,363				3,586				3,363	3,586
Loans & Leases [Abstract]
Ending Balance	89,680				71,721				89,680	71,721
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	89,680				71,721				89,680	71,721
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance				$ 1,449				$ 1,546	1,449	1,546
Charge-Offs									0	0
Recoveries									0	0
Provision									(27)	(97)
Ending Balance	1,422				1,449				1,422	1,449	$ 1,546
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	1,422				1,449				1,422	1,449
Loans & Leases [Abstract]
Ending Balance	0				0				0	0
Ending Balance Individually Evaluated for Impairment	0				0				0	0
Ending Balance Collectively Evaluated for Impairment	0				0				0	0
Restructured Loans [Member]
Loans & Leases [Abstract]
Ending Balance Individually Evaluated for Impairment	$ 3,000				$ 3,300				$ 3,000	$ 3,300