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, 2019	Mar. 31, 2018	Dec. 31, 2018
Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 55,266	$ 50,342	$ 50,342
Charge-Offs	(400)	(21)	(767)
Recoveries	41	23	158
Provision	0	333	5,533
Ending Balance	54,907	50,677	55,266
Ending Balance Individually Evaluated for Impairment	627	641	565
Ending Balance Collectively Evaluated for Impairment	54,280	50,036	54,701
Loans & Leases [Abstract]
Ending Balance	2,563,968	2,235,083	2,571,241
Ending Balance Individually Evaluated for Impairment	16,397	9,265	16,347
Ending Balance Collectively Evaluated for Impairment	2,547,571	2,225,818	2,554,894
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,609	10,922	10,922
Charge-Offs	0	0	0
Recoveries	0	0	2
Provision	(209)	156	685
Ending Balance	11,400	11,078	11,609
Ending Balance Individually Evaluated for Impairment	214	335	234
Ending Balance Collectively Evaluated for Impairment	11,186	10,743	11,375
Loans & Leases [Abstract]
Ending Balance	815,366	698,349	826,549
Ending Balance Individually Evaluated for Impairment	4,638	4,784	4,676
Ending Balance Collectively Evaluated for Impairment	810,728	693,565	821,873
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	14,092	12,085	12,085
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	401	157	2,007
Ending Balance	14,493	12,242	14,092
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	14,493	12,242	14,092
Loans & Leases [Abstract]
Ending Balance	603,601	508,400	584,625
Ending Balance Individually Evaluated for Impairment	7,238	0	7,238
Ending Balance Collectively Evaluated for Impairment	596,363	508,400	577,387
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,249	1,846	1,846
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	(42)	27	(597)
Ending Balance	1,207	1,873	1,249
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,207	1,873	1,249
Loans & Leases [Abstract]
Ending Balance	99,837	96,315	98,568
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	99,837	96,315	98,568
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	880	815	815
Charge-Offs	0	0	(31)
Recoveries	3	3	15
Provision	(20)	9	81
Ending Balance	863	827	880
Ending Balance Individually Evaluated for Impairment	124	76	125
Ending Balance Collectively Evaluated for Impairment	739	751	755
Loans & Leases [Abstract]
Ending Balance	258,359	264,137	259,736
Ending Balance Individually Evaluated for Impairment	2,469	2,427	2,491
Ending Balance Collectively Evaluated for Impairment	255,890	261,710	257,245
Home Equity Lines & Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,761	2,324	2,324
Charge-Offs	0	(4)	(8)
Recoveries	6	1	6
Provision	(29)	22	439
Ending Balance	2,738	2,343	2,761
Ending Balance Individually Evaluated for Impairment	13	17	15
Ending Balance Collectively Evaluated for Impairment	2,725	2,326	2,746
Loans & Leases [Abstract]
Ending Balance	40,072	34,691	40,789
Ending Balance Individually Evaluated for Impairment	255	332	297
Ending Balance Collectively Evaluated for Impairment	39,817	34,359	40,492
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	8,242	8,159	8,159
Charge-Offs	0	0	0
Recoveries	6	6	61
Provision	(800)	(297)	22
Ending Balance	7,448	7,868	8,242
Ending Balance Individually Evaluated for Impairment	98	0	0
Ending Balance Collectively Evaluated for Impairment	7,350	7,868	8,242
Loans & Leases [Abstract]
Ending Balance	257,004	261,427	290,463
Ending Balance Individually Evaluated for Impairment	200	0	0
Ending Balance Collectively Evaluated for Impairment	256,804	261,427	290,463
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,656	9,197	9,197
Charge-Offs	(379)	0	(613)
Recoveries	12	2	20
Provision	644	175	3,052
Ending Balance	11,933	9,374	11,656
Ending Balance Individually Evaluated for Impairment	172	205	185
Ending Balance Collectively Evaluated for Impairment	11,761	9,169	11,471
Loans & Leases [Abstract]
Ending Balance	364,439	274,682	343,834
Ending Balance Individually Evaluated for Impairment	1,591	1,713	1,639
Ending Balance Collectively Evaluated for Impairment	362,848	272,969	342,195
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	494	209	209
Charge-Offs	(21)	(17)	(115)
Recoveries	14	11	54
Provision	(20)	36	346
Ending Balance	467	239	494
Ending Balance Individually Evaluated for Impairment	6	8	6
Ending Balance Collectively Evaluated for Impairment	461	231	488
Loans & Leases [Abstract]
Ending Balance	18,418	6,685	19,412
Ending Balance Individually Evaluated for Impairment	6	9	6
Ending Balance Collectively Evaluated for Impairment	18,412	6,676	19,406
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,022	3,363	3,363
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	(14)	27	659
Ending Balance	4,008	3,390	4,022
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	4,008	3,390	4,022
Loans & Leases [Abstract]
Ending Balance	106,872	90,397	107,265
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	106,872	90,397	107,265
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	261	1,422	1,422
Charge-Offs	0	0	0
Recoveries	0	0	0
Provision	89	21	(1,161)
Ending Balance	350	1,443	261
Ending Balance Individually Evaluated for Impairment	0	0	0
Ending Balance Collectively Evaluated for Impairment	350	1,443	261
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	$ 2,700	$ 2,900	$ 2,800

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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- Definition

Amount of expense related to estimated loss from loan and lease transactions.

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