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		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 50,342	$ 47,919	$ 47,919
Charge-Offs	(21)	(151)	(685)
Recoveries	23	32	258
Provision	333	600	2,850
Ending Balance	50,677	48,400	50,342
Ending Balance Individually Evaluated for Impairment	641	1,162	684
Ending Balance Collectively Evaluated for Impairment	50,036	47,238	49,658
Loans & Leases [Abstract]
Ending Balance	2,235,083	2,146,032	2,215,295
Ending Balance Individually Evaluated for Impairment	9,265	10,759	9,279
Ending Balance Collectively Evaluated for Impairment	2,225,818	2,135,273	2,206,016
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	10,922	11,110	11,110
Charge-Offs	0	(109)	(109)
Recoveries	0	7	109
Provision	156	657	(188)
Ending Balance	11,078	11,665	10,922
Ending Balance Individually Evaluated for Impairment	335	471	366
Ending Balance Collectively Evaluated for Impairment	10,743	11,194	10,556
Loans & Leases [Abstract]
Ending Balance	698,349	676,527	684,961
Ending Balance Individually Evaluated for Impairment	4,784	4,924	4,822
Ending Balance Collectively Evaluated for Impairment	693,565	671,603	680,139
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	12,085	9,450	9,450
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	157	(57)	2,635
Ending Balance	12,242	9,393	12,085
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	12,242	9,393	12,085
Loans & Leases [Abstract]
Ending Balance	508,400	463,645	499,231
Ending Balance Individually Evaluated for Impairment	0	975	0
Ending Balance Collectively Evaluated for Impairment	508,400	462,670	499,231
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,846	3,223	3,223
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	27	(277)	(1,377)
Ending Balance	1,873	2,946	1,846
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,873	2,946	1,846
Loans & Leases [Abstract]
Ending Balance	96,315	164,791	100,206
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	96,315	164,791	100,206
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	815	865	865
Charge-Offs	0	0	(53)
Recoveries	3	3	40
Provision	9	12	(37)
Ending Balance	827	880	815
Ending Balance Individually Evaluated for Impairment	76	69	73
Ending Balance Collectively Evaluated for Impairment	751	811	742
Loans & Leases [Abstract]
Ending Balance	264,137	249,628	260,751
Ending Balance Individually Evaluated for Impairment	2,427	2,069	2,373
Ending Balance Collectively Evaluated for Impairment	261,710	247,559	258,378
Home Equity Lines & Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,324	2,140	2,140
Charge-Offs	(4)	0	(3)
Recoveries	1	2	8
Provision	22	(17)	179
Ending Balance	2,343	2,125	2,324
Ending Balance Individually Evaluated for Impairment	17	17	17
Ending Balance Collectively Evaluated for Impairment	2,326	2,108	2,307
Loans & Leases [Abstract]
Ending Balance	34,691	31,817	34,525
Ending Balance Individually Evaluated for Impairment	332	451	340
Ending Balance Collectively Evaluated for Impairment	34,359	31,366	34,185
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	8,159	7,381	7,381
Charge-Offs	0	(7)	(374)
Recoveries	6	0	17
Provision	(297)	(863)	1,135
Ending Balance	7,868	6,511	8,159
Ending Balance Individually Evaluated for Impairment	0	331	0
Ending Balance Collectively Evaluated for Impairment	7,868	6,180	8,159
Loans & Leases [Abstract]
Ending Balance	261,427	266,010	273,582
Ending Balance Individually Evaluated for Impairment	0	698	0
Ending Balance Collectively Evaluated for Impairment	261,427	265,312	273,582
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,197	8,515	8,515
Charge-Offs	0	0	0
Recoveries	2	4	8
Provision	175	(40)	674
Ending Balance	9,374	8,479	9,197
Ending Balance Individually Evaluated for Impairment	205	268	220
Ending Balance Collectively Evaluated for Impairment	9,169	8,211	8,977
Loans & Leases [Abstract]
Ending Balance	274,682	209,184	265,703
Ending Balance Individually Evaluated for Impairment	1,713	1,633	1,734
Ending Balance Collectively Evaluated for Impairment	272,969	207,551	263,969
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	209	200	200
Charge-Offs	(17)	(35)	(146)
Recoveries	11	16	76
Provision	36	17	79
Ending Balance	239	198	209
Ending Balance Individually Evaluated for Impairment	8	6	8
Ending Balance Collectively Evaluated for Impairment	231	192	201
Loans & Leases [Abstract]
Ending Balance	6,685	7,119	6,656
Ending Balance Individually Evaluated for Impairment	9	9	10
Ending Balance Collectively Evaluated for Impairment	6,676	7,110	6,646
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	3,363	3,586	3,586
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	27	280	(223)
Ending Balance	3,390	3,866	3,363
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	3,390	3,866	3,363
Loans & Leases [Abstract]
Ending Balance	90,397	77,311	89,680
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	90,397	77,311	89,680
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,422	1,449	1,449
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	21	888	(27)
Ending Balance	1,443	2,337	1,422
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,443	2,337	1,422
Loans & Leases [Abstract]
Ending Balance	0	0	0
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	0	0	0
Restructured Loans [Member]
Loans & Leases [Abstract]
Ending Balance Individually Evaluated for Impairment	$ 2,900	$ 3,400	$ 3,000