Loans & Leases and Allowance for Credit Losses, Allocation of Allowance For Credit Losses by Portfolio Segment and By Impairment Methodology (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 50,677	$ 48,400	$ 50,342	$ 47,919	$ 47,919
Charge-Offs	(67)	(25)	(88)	(176)	(685)
Recoveries	27	39	50	71	258
Provision	500	650	833	1,250	2,850
Ending Balance	51,137	49,064	51,137	49,064	50,342
Ending Balance Individually Evaluated for Impairment	974	1,138	974	1,138	684
Ending Balance Collectively Evaluated for Impairment	50,163	47,926	50,163	47,926	49,658
Loans & Leases [Abstract]
Ending Balance	2,344,448	2,204,082	2,344,448	2,204,082	2,215,295
Ending Balance Individually Evaluated for Impairment	9,998	10,444	9,998	10,444	9,279
Ending Balance Collectively Evaluated for Impairment	2,334,450	2,193,638	2,334,450	2,193,638	2,206,016
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,078	11,665	10,922	11,110	11,110
Charge-Offs	0	0	0	(109)	(109)
Recoveries	0	3	0	10	109
Provision	(295)	(426)	(139)	231	(188)
Ending Balance	10,783	11,242	10,783	11,242	10,922
Ending Balance Individually Evaluated for Impairment	333	468	333	468	366
Ending Balance Collectively Evaluated for Impairment	10,450	10,774	10,450	10,774	10,556
Loans & Leases [Abstract]
Ending Balance	752,043	670,591	752,043	670,591	684,961
Ending Balance Individually Evaluated for Impairment	4,749	4,890	4,749	4,890	4,822
Ending Balance Collectively Evaluated for Impairment	747,294	665,701	747,294	665,701	680,139
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	12,242	9,393	12,085	9,450	9,450
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	1,072	872	1,229	815	2,635
Ending Balance	13,314	10,265	13,314	10,265	12,085
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	13,314	10,265	13,314	10,265	12,085
Loans & Leases [Abstract]
Ending Balance	540,852	473,153	540,852	473,153	499,231
Ending Balance Individually Evaluated for Impairment	0	976	0	976	0
Ending Balance Collectively Evaluated for Impairment	540,852	472,177	540,852	472,177	499,231
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,873	2,946	1,846	3,223	3,223
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	(257)	(259)	(230)	(536)	(1,377)
Ending Balance	1,616	2,687	1,616	2,687	1,846
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,616	2,687	1,616	2,687	1,846
Loans & Leases [Abstract]
Ending Balance	94,223	168,487	94,223	168,487	100,206
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	94,223	168,487	94,223	168,487	100,206
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	827	880	815	865	865
Charge-Offs	(12)	0	(12)	0	(53)
Recoveries	3	16	6	19	40
Provision	46	(24)	55	(12)	(37)
Ending Balance	864	872	864	872	815
Ending Balance Individually Evaluated for Impairment	131	61	131	61	73
Ending Balance Collectively Evaluated for Impairment	733	811	733	811	742
Loans & Leases [Abstract]
Ending Balance	261,804	252,653	261,804	252,653	260,751
Ending Balance Individually Evaluated for Impairment	2,628	1,894	2,628	1,894	2,373
Ending Balance Collectively Evaluated for Impairment	259,176	250,759	259,176	250,759	258,378
Home Equity Lines & Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,343	2,125	2,324	2,140	2,140
Charge-Offs	0	0	(4)	0	(3)
Recoveries	1	3	2	5	8
Provision	204	42	226	25	179
Ending Balance	2,548	2,170	2,548	2,170	2,324
Ending Balance Individually Evaluated for Impairment	16	15	16	15	17
Ending Balance Collectively Evaluated for Impairment	2,532	2,155	2,532	2,155	2,307
Loans & Leases [Abstract]
Ending Balance	37,669	32,174	37,669	32,174	34,525
Ending Balance Individually Evaluated for Impairment	324	344	324	344	340
Ending Balance Collectively Evaluated for Impairment	37,345	31,830	37,345	31,830	34,185
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	7,868	6,511	8,159	7,381	7,381
Charge-Offs	0	0	0	(7)	(374)
Recoveries	7	0	13	0	17
Provision	(217)	725	(514)	(138)	1,135
Ending Balance	7,658	7,236	7,658	7,236	8,159
Ending Balance Individually Evaluated for Impairment	0	322	0	322	0
Ending Balance Collectively Evaluated for Impairment	7,658	6,914	7,658	6,914	8,159
Loans & Leases [Abstract]
Ending Balance	273,170	265,899	273,170	265,899	273,582
Ending Balance Individually Evaluated for Impairment	0	688	0	688	0
Ending Balance Collectively Evaluated for Impairment	273,170	265,211	273,170	265,211	273,582
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,374	8,479	9,197	8,515	8,515
Charge-Offs	(14)	0	(14)	0	0
Recoveries	1	0	3	4	8
Provision	75	1,065	250	1,025	674
Ending Balance	9,436	9,544	9,436	9,544	9,197
Ending Balance Individually Evaluated for Impairment	487	266	487	266	220
Ending Balance Collectively Evaluated for Impairment	8,949	9,278	8,949	9,278	8,977
Loans & Leases [Abstract]
Ending Balance	286,651	254,887	286,651	254,887	265,703
Ending Balance Individually Evaluated for Impairment	2,290	1,624	2,290	1,624	1,734
Ending Balance Collectively Evaluated for Impairment	284,361	253,263	284,361	253,263	263,969
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	239	198	209	200	200
Charge-Offs	(41)	(25)	(58)	(60)	(146)
Recoveries	15	17	26	33	76
Provision	57	15	93	32	79
Ending Balance	270	205	270	205	209
Ending Balance Individually Evaluated for Impairment	7	6	7	6	8
Ending Balance Collectively Evaluated for Impairment	263	199	263	199	201
Loans & Leases [Abstract]
Ending Balance	7,390	7,533	7,390	7,533	6,656
Ending Balance Individually Evaluated for Impairment	7	28	7	28	10
Ending Balance Collectively Evaluated for Impairment	7,383	7,505	7,383	7,505	6,646
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	3,390	3,866	3,363	3,586	3,586
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	10	(914)	37	(634)	(223)
Ending Balance	3,400	2,952	3,400	2,952	3,363
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	3,400	2,952	3,400	2,952	3,363
Loans & Leases [Abstract]
Ending Balance	90,646	78,705	90,646	78,705	89,680
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	90,646	78,705	90,646	78,705	89,680
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,443	2,337	1,422	1,449	1,449
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	(195)	(446)	(174)	442	(27)
Ending Balance	1,248	1,891	1,248	1,891	1,422
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,248	1,891	1,248	1,891	1,422
Loans & Leases [Abstract]
Ending Balance	0	0	0	0	0
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	0	0	0	0	0
Restructured Loans [Member]
Loans & Leases [Abstract]
Ending Balance Individually Evaluated for Impairment	$ 2,900	$ 3,200	$ 2,900	$ 3,200	$ 3,000