Loans & Leases and Allowance for Credit Losses, Allocation of Allowance For Credit Losses by Portfolio Segment and By Impairment Methodology (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 55,058,000	$ 55,125,000	$ 55,012,000	$ 55,266,000	$ 55,266,000
Charge-Offs	(25,000)	(235,000)	(487,000)	(654,000)	(675,000)
Recoveries	65,000	64,000	273,000	142,000	221,000
Provision	1,700,000	0	2,000,000	200,000	200,000
Ending Balance	56,798,000	54,954,000	56,798,000	54,954,000	55,012,000
Ending Balance Individually Evaluated for Impairment	287,000	633,000	287,000	633,000	661,000
Ending Balance Collectively Evaluated for Impairment	56,511,000	54,321,000	56,511,000	54,321,000	54,351,000
Loans & Leases [Abstract]
Ending Balance	3,111,931,000	2,617,122,000	3,111,931,000	2,617,122,000	2,673,027,000
Ending Balance Individually Evaluated for Impairment	9,218,000	14,635,000	9,218,000	14,635,000	14,691,000
Ending Balance Collectively Evaluated for Impairment	3,102,713,000	2,602,487,000	3,102,713,000	2,602,487,000	2,658,336,000
Commercial Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	21,423,000	10,680,000	11,053,000	11,609,000	11,609,000
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	2,866,000	82,000	13,236,000	(847,000)	(556,000)
Ending Balance	24,289,000	10,762,000	24,289,000	10,762,000	11,053,000
Ending Balance Individually Evaluated for Impairment	0	253,000	0	253,000	234,000
Ending Balance Collectively Evaluated for Impairment	24,289,000	10,509,000	24,289,000	10,509,000	10,819,000
Loans & Leases [Abstract]
Ending Balance	871,623,000	816,668,000	871,623,000	816,668,000	838,570,000
Ending Balance Individually Evaluated for Impairment	108,000	4,563,000	108,000	4,563,000	4,524,000
Ending Balance Collectively Evaluated for Impairment	871,515,000	812,105,000	871,515,000	812,105,000	834,046,000
Agricultural Real Estate [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	9,021,000	14,572,000	15,128,000	14,092,000	14,092,000
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	(229,000)	251,000	(6,336,000)	731,000	1,036,000
Ending Balance	8,792,000	14,823,000	8,792,000	14,823,000	15,128,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	8,792,000	14,823,000	8,792,000	14,823,000	15,128,000
Loans & Leases [Abstract]
Ending Balance	639,172,000	617,310,000	639,172,000	617,310,000	625,767,000
Ending Balance Individually Evaluated for Impairment	5,629,000	5,678,000	5,629,000	5,678,000	5,654,000
Ending Balance Collectively Evaluated for Impairment	633,543,000	611,632,000	633,543,000	611,632,000	620,113,000
Real Estate Construction [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,452,000	1,697,000	1,949,000	1,249,000	1,249,000
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	116,000	32,000	(381,000)	480,000	700,000
Ending Balance	1,568,000	1,729,000	1,568,000	1,729,000	1,949,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,568,000	1,729,000	1,568,000	1,729,000	1,949,000
Loans & Leases [Abstract]
Ending Balance	186,623,000	98,662,000	186,623,000	98,662,000	115,644,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	186,623,000	98,662,000	186,623,000	98,662,000	115,644,000
Residential 1st Mortgages [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,771,000	864,000	855,000	880,000	880,000
Charge-Offs	0	0	0	0	0
Recoveries	3,000	2,000	49,000	8,000	13,000
Provision	(825,000)	(11,000)	45,000	(33,000)	(38,000)
Ending Balance	949,000	855,000	949,000	855,000	855,000
Ending Balance Individually Evaluated for Impairment	118,000	121,000	118,000	121,000	118,000
Ending Balance Collectively Evaluated for Impairment	831,000	734,000	831,000	734,000	737,000
Loans & Leases [Abstract]
Ending Balance	293,489,000	255,394,000	293,489,000	255,394,000	255,253,000
Ending Balance Individually Evaluated for Impairment	2,390,000	2,422,000	2,390,000	2,422,000	2,368,000
Ending Balance Collectively Evaluated for Impairment	291,099,000	252,972,000	291,099,000	252,972,000	252,885,000
Home Equity Lines and Loans [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,239,000	2,743,000	2,675,000	2,761,000	2,761,000
Charge-Offs	0	0	(7,000)	0	0
Recoveries	31,000	9,000	65,000	20,000	28,000
Provision	(154,000)	(50,000)	(617,000)	(79,000)	(114,000)
Ending Balance	2,116,000	2,702,000	2,116,000	2,702,000	2,675,000
Ending Balance Individually Evaluated for Impairment	8,000	12,000	8,000	12,000	12,000
Ending Balance Collectively Evaluated for Impairment	2,108,000	2,690,000	2,108,000	2,690,000	2,663,000
Loans & Leases [Abstract]
Ending Balance	35,875,000	39,490,000	35,875,000	39,490,000	39,270,000
Ending Balance Individually Evaluated for Impairment	164,000	237,000	164,000	237,000	229,000
Ending Balance Collectively Evaluated for Impairment	35,711,000	39,253,000	35,711,000	39,253,000	39,041,000
Agricultural [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,790,000	7,481,000	8,076,000	8,242,000	8,242,000
Charge-Offs	0	0	0	0	0
Recoveries	24,000	23,000	54,000	34,000	38,000
Provision	(142,000)	341,000	(3,458,000)	(431,000)	(204,000)
Ending Balance	4,672,000	7,845,000	4,672,000	7,845,000	8,076,000
Ending Balance Individually Evaluated for Impairment	92,000	95,000	92,000	95,000	99,000
Ending Balance Collectively Evaluated for Impairment	4,580,000	7,750,000	4,580,000	7,750,000	7,977,000
Loans & Leases [Abstract]
Ending Balance	252,031,000	289,182,000	252,031,000	289,182,000	292,904,000
Ending Balance Individually Evaluated for Impairment	498,000	192,000	498,000	192,000	188,000
Ending Balance Collectively Evaluated for Impairment	251,533,000	288,990,000	251,533,000	288,990,000	292,716,000
Commercial [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	10,043,000	12,267,000	11,466,000	11,656,000	11,656,000
Charge-Offs	0	(213,000)	(426,000)	(592,000)	(592,000)
Recoveries	0	17,000	80,000	40,000	90,000
Provision	(50,000)	(366,000)	(1,127,000)	601,000	312,000
Ending Balance	9,993,000	11,705,000	9,993,000	11,705,000	11,466,000
Ending Balance Individually Evaluated for Impairment	13,000	147,000	13,000	147,000	137,000
Ending Balance Collectively Evaluated for Impairment	9,980,000	11,558,000	9,980,000	11,558,000	11,329,000
Loans & Leases [Abstract]
Ending Balance	367,052,000	381,774,000	367,052,000	381,774,000	384,795,000
Ending Balance Individually Evaluated for Impairment	235,000	1,538,000	235,000	1,538,000	1,528,000
Ending Balance Collectively Evaluated for Impairment	366,817,000	380,236,000	366,817,000	380,236,000	383,267,000
Consumer & Other [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	359,000	464,000	456,000	494,000	494,000
Charge-Offs	(25,000)	(22,000)	(54,000)	(62,000)	(83,000)
Recoveries	7,000	13,000	25,000	40,000	52,000
Provision	23,000	20,000	(63,000)	3,000	(7,000)
Ending Balance	364,000	475,000	364,000	475,000	456,000
Ending Balance Individually Evaluated for Impairment	56,000	5,000	56,000	5,000	61,000
Ending Balance Collectively Evaluated for Impairment	308,000	470,000	308,000	470,000	395,000
Loans & Leases [Abstract]
Ending Balance	359,697,000	16,871,000	359,697,000	16,871,000	15,422,000
Ending Balance Individually Evaluated for Impairment	194,000	5,000	194,000	5,000	200,000
Ending Balance Collectively Evaluated for Impairment	359,503,000	16,866,000	359,503,000	16,866,000	15,222,000
Leases [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,800,000	3,100,000	3,162,000	4,022,000	4,022,000
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	114,000	(47,000)	(248,000)	(969,000)	(860,000)
Ending Balance	2,914,000	3,053,000	2,914,000	3,053,000	3,162,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	2,914,000	3,053,000	2,914,000	3,053,000	3,162,000
Loans & Leases [Abstract]
Ending Balance	106,369,000	101,771,000	106,369,000	101,771,000	105,402,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	106,369,000	101,771,000	106,369,000	101,771,000	105,402,000
Unallocated [Member]
Allowance for Credit Losses [Roll Forward]
Beginning Balance	1,160,000	1,257,000	192,000	261,000	261,000
Charge-Offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision	(19,000)	(252,000)	949,000	744,000	(69,000)
Ending Balance	1,141,000	1,005,000	1,141,000	1,005,000	192,000
Ending Balance Individually Evaluated for Impairment	0	0	0	0	0
Ending Balance Collectively Evaluated for Impairment	1,141,000	1,005,000	1,141,000	1,005,000	192,000
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	$ 828,500	$ 2,600,000	$ 828,500	$ 2,600,000	$ 2,600,000