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		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 51,137	$ 49,064	$ 50,342	$ 47,919	$ 47,919
Charge-Offs	(634)	(54)	(723)	(230)	(685)
Recoveries	64	134	115	205	258
Provision	2,500	1,600	3,333	2,850	2,850
Ending Balance	53,067	50,744	53,067	50,744	50,342
Ending Balance Individually Evaluated for Impairment	649	768	649	768	684
Ending Balance Collectively Evaluated for Impairment	52,418	49,976	52,418	49,976	49,658
Loans & Leases [Abstract]
Ending Balance	2,416,602	2,216,779	2,416,602	2,216,779	2,215,295
Ending Balance Individually Evaluated for Impairment	16,359	9,238	16,359	9,238	9,279
Ending Balance Collectively Evaluated for Impairment	2,400,243	2,207,541	2,400,243	2,207,541	2,206,016
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	10,783	11,242	10,922	11,110	11,110
Charge-Offs	0	0	0	(109)	(109)
Recoveries	0	99	0	110	109
Provision	108	(244)	(31)	(14)	(188)
Ending Balance	10,891	11,097	10,891	11,097	10,922
Ending Balance Individually Evaluated for Impairment	314	385	314	385	366
Ending Balance Collectively Evaluated for Impairment	10,577	10,712	10,577	10,712	10,556
Loans & Leases [Abstract]
Ending Balance	767,410	683,037	767,410	683,037	684,961
Ending Balance Individually Evaluated for Impairment	4,713	4,855	4,713	4,855	4,822
Ending Balance Collectively Evaluated for Impairment	762,697	678,182	762,697	678,182	680,139
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	13,314	10,265	12,085	9,450	9,450
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	193	1,131	1,422	1,946	2,635
Ending Balance	13,507	11,396	13,507	11,396	12,085
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	13,507	11,396	13,507	11,396	12,085
Loans & Leases [Abstract]
Ending Balance	553,608	470,738	553,608	470,738	499,231
Ending Balance Individually Evaluated for Impairment	7,238	0	7,238	0	0
Ending Balance Collectively Evaluated for Impairment	546,370	470,738	546,370	470,738	499,231
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,616	2,687	1,846	3,223	3,223
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	(142)	63	(372)	(473)	(1,377)
Ending Balance	1,474	2,750	1,474	2,750	1,846
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,474	2,750	1,474	2,750	1,846
Loans & Leases [Abstract]
Ending Balance	92,521	162,167	92,521	162,167	100,206
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	92,521	162,167	92,521	162,167	100,206
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	864	872	815	865	865
Charge-Offs	0	0	(12)	0	(53)
Recoveries	6	18	12	37	40
Provision	4	107	59	95	(37)
Ending Balance	874	997	874	997	815
Ending Balance Individually Evaluated for Impairment	121	61	121	61	73
Ending Balance Collectively Evaluated for Impairment	753	936	753	936	742
Loans & Leases [Abstract]
Ending Balance	263,549	257,920	263,549	257,920	260,751
Ending Balance Individually Evaluated for Impairment	2,426	1,887	2,426	1,887	2,373
Ending Balance Collectively Evaluated for Impairment	261,123	256,033	261,123	256,033	258,378
Home Equity Lines & Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,548	2,170	2,324	2,140	2,140
Charge-Offs	(10)	0	(14)	0	(3)
Recoveries	2	1	4	6	8
Provision	58	90	284	115	179
Ending Balance	2,598	2,261	2,598	2,261	2,324
Ending Balance Individually Evaluated for Impairment	15	17	15	17	17
Ending Balance Collectively Evaluated for Impairment	2,583	2,244	2,583	2,244	2,307
Loans & Leases [Abstract]
Ending Balance	38,490	33,350	38,490	33,350	34,525
Ending Balance Individually Evaluated for Impairment	305	369	305	369	340
Ending Balance Collectively Evaluated for Impairment	38,185	32,981	38,185	32,981	34,185
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	7,658	7,236	8,159	7,381	7,381
Charge-Offs	0	0	0	(7)	(374)
Recoveries	27	0	40	0	17
Provision	518	124	4	(14)	1,135
Ending Balance	8,203	7,360	8,203	7,360	8,159
Ending Balance Individually Evaluated for Impairment	0	69	0	69	0
Ending Balance Collectively Evaluated for Impairment	8,203	7,291	8,203	7,291	8,159
Loans & Leases [Abstract]
Ending Balance	287,821	259,127	287,821	259,127	273,582
Ending Balance Individually Evaluated for Impairment	0	368	0	368	0
Ending Balance Collectively Evaluated for Impairment	287,821	258,759	287,821	258,759	273,582
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,436	9,544	9,197	8,515	8,515
Charge-Offs	(599)	0	(613)	0	0
Recoveries	16	2	19	6	8
Provision	779	(486)	1,029	539	674
Ending Balance	9,632	9,060	9,632	9,060	9,197
Ending Balance Individually Evaluated for Impairment	192	231	192	231	220
Ending Balance Collectively Evaluated for Impairment	9,440	8,829	9,440	8,829	8,977
Loans & Leases [Abstract]
Ending Balance	304,333	257,951	304,333	257,951	265,703
Ending Balance Individually Evaluated for Impairment	1,670	1,752	1,670	1,752	1,734
Ending Balance Collectively Evaluated for Impairment	302,663	256,199	302,663	256,199	263,969
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	270	205	209	200	200
Charge-Offs	(25)	(54)	(84)	(114)	(146)
Recoveries	13	14	40	46	76
Provision	49	44	142	77	79
Ending Balance	307	209	307	209	209
Ending Balance Individually Evaluated for Impairment	7	5	7	5	8
Ending Balance Collectively Evaluated for Impairment	300	204	300	204	201
Loans & Leases [Abstract]
Ending Balance	7,723	7,312	7,723	7,312	6,656
Ending Balance Individually Evaluated for Impairment	7	7	7	7	10
Ending Balance Collectively Evaluated for Impairment	7,716	7,305	7,716	7,305	6,646
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	3,400	2,952	3,363	3,586	3,586
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	393	242	430	(392)	(223)
Ending Balance	3,793	3,194	3,793	3,194	3,363
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	3,793	3,194	3,793	3,194	3,363
Loans & Leases [Abstract]
Ending Balance	101,147	85,177	101,147	85,177	89,680
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	101,147	85,177	101,147	85,177	89,680
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,248	1,891	1,422	1,449	1,449
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	540	529	366	971	(27)
Ending Balance	1,788	2,420	1,788	2,420	1,422
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,788	2,420	1,788	2,420	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	$ 2,800	$ 3,000	$ 2,800	$ 3,000	$ 3,000