Note 4 - Regulatory Assets and Liabilities - Amount of Regulatory Assets and Liabilities Recorded on Consolidated Balance Sheets (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2020	Dec. 31, 2019
Regulatory Assets - Current		$ 19,958	$ 21,650
Regulatory Assets - Long -Term		141,063	144,138
Regulatory Assets - Total		161,021	165,788
Regulatory Liabilities - Current		13,023	7,480
Regulatory Liabilities - Long -Term		238,160	239,906
Regulatory Liabilities - Total		251,183	247,386
Net Regulatory Asset Position - Current		6,935	14,170
Net Regulatory Asset Position - Long-Term		(97,097)	(95,768)
Net Regulatory Asset/(Liability) Position		(90,162)	(81,598)
Deferred Income Taxes [Member]
Regulatory Liabilities - Current		0	0
Regulatory Liabilities - Long -Term		138,517	141,707
Regulatory Liabilities - Total		138,517	141,707
Accumulated Reserve for Estimated Removal Costs - Net of Salvage [Member]
Regulatory Liabilities - Current		0	0
Regulatory Liabilities - Long -Term		99,055	97,726
Regulatory Liabilities - Total		99,055	97,726
Refundable Fuel Clause Adjustment Revenues [Member]
Regulatory Liabilities - Current		10,241	3,982
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 10,241	$ 3,982
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months	12 months
North Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 1,010	$ 700
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 1,010	$ 700
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		6 months	12 months
South Dakota Phase-in Rate Plan Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 783	$ 355
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 783	$ 355
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		2 months	9 months
Revenue for Rate Case Expenses Subject to Refund - Minnesota [Member]
Regulatory Liabilities - Current		$ 0	$ 0
Regulatory Liabilities - Long -Term		518	401
Regulatory Liabilities - Total		518	401
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits [Member]
Regulatory Liabilities - Current		471	471
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 471	$ 471
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		12 months	12 months
Minnesota Energy Intensive Trade Exposed Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 198	$ 164
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 198	$ 164
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		3 months	12 months
South Dakota Transmission Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 159
Regulatory Liabilities - Long -Term		0
Regulatory Liabilities - Total		$ 159
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		8 months
North Dakota Renewable Resource Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current		$ 156	$ 1,515
Regulatory Liabilities - Long -Term		0	0
Regulatory Liabilities - Total		$ 156	$ 1,515
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		9 months	12 months
North Dakota Generation Cost Recovery Rider Accrued Refund [Member]
Regulatory Liabilities - Current			$ 287
Regulatory Liabilities - Long -Term			0
Regulatory Liabilities - Total			$ 287
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)			6 months
Other [Member]
Regulatory Liabilities - Current		$ 5	$ 6
Regulatory Liabilities - Long -Term		70	72
Regulatory Liabilities - Total		$ 75	$ 78
Regulatory Liabilities - Remaining Recovery/Refund Period (Month)		162 months	168 months
Prior Service Costs and Actuarial Losses on Pensions and Other Postretirement Benefits [Member]
Regulatory Assets - Current	[1]	$ 9,018	$ 9,090
Regulatory Assets - Long -Term	[1]	124,901	129,102
Regulatory Assets - Total	[1]	133,919	138,192
Accumulated Asset Retirement Obligation (ARO) Accretion/Depreciation Adjustment [Member]
Regulatory Assets - Current	[1]	0	0
Regulatory Assets - Long -Term	[1]	8,243	7,772
Regulatory Assets - Total	[2]	8,243	7,772
Minnesota Transmission Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	6,563	4,208
Regulatory Assets - Long -Term	[2]	0	0
Regulatory Assets - Total	[2]	$ 6,563	$ 4,208
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	12 months	12 months
Conservation Improvement Program Costs and Incentives [Member]
Regulatory Assets - Current	[2]	$ 276	$ 4,024
Regulatory Assets - Long -Term	[2]	3,553	2,844
Regulatory Assets - Total	[2]	$ 3,829	$ 6,868
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	27 months	21 months
MISO Schedule 26/26A Transmission Cost Recovery Rider True-up [Member]
Regulatory Assets - Current	[1]	$ 1,500	$ 2,033
Regulatory Assets - Long -Term	[1]	963	968
Regulatory Assets - Total	[1]	$ 2,463	$ 3,001
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	30 months	24 months
Nonservice Costs Components of Postretirement Benefits Capitalized for Ratemaking Purposes and Subject to Deferred Recovery [Member]
Regulatory Assets - Current	[1]	$ 0	$ 0
Regulatory Assets - Long -Term	[1]	1,974	1,681
Regulatory Assets - Total	[1]	1,974	1,681
Minnesota Renewable Resource Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	722	131
Regulatory Assets - Long -Term	[2]	0	0
Regulatory Assets - Total	[2]	$ 722	$ 131
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	12 months	12 months
Debt Reacquisition Premiums [Member]
Regulatory Assets - Current	[1]	$ 204	$ 201
Regulatory Assets - Long -Term	[1]	430	548
Regulatory Assets - Total	[1]	$ 634	$ 749
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	147 months	153 months
Big Stone II Unrecovered Project Costs - Minnesota [Member]
Regulatory Assets - Current	[1]	$ 583	$ 715
Regulatory Assets - Long -Term	[1]	0	225
Regulatory Assets - Total	[1]	$ 583	$ 940
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	10 months	16 months
Minnesota SPP Transmission Cost Recovery Tracker [Member]
Regulatory Assets - Current	[1]	$ 0	$ 0
Regulatory Assets - Long -Term	[1]	401	202
Regulatory Assets - Total	[1]	401	202
Deferred Marked-to-Market Losses [Member]
Regulatory Assets - Current	[1]	372	743
Regulatory Assets - Total	[1]	$ 372	$ 743
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	6 months	12 months
Big Stone II Unrecovered Project Costs - South Dakota [Member]
Regulatory Assets - Current	[1]	$ 144	$ 144
Regulatory Assets - Long -Term	[1]	180	253
Regulatory Assets - Total	[1]	$ 324	$ 397
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	27 months	33 months
South Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 138	$ 138
Regulatory Assets - Long -Term	[1]	177	245
Regulatory Assets - Total	[1]	$ 315	$ 383
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	28 months	34 months
North Dakota Generation Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 312
Regulatory Assets - Long -Term	[2]	0
Regulatory Assets - Total	[2]	$ 312
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	12 months
North Dakota Deferred Rate Case Expenses Subject to Recovery [Member]
Regulatory Assets - Current	[1]	$ 122	$ 122
Regulatory Assets - Long -Term	[1]	183	244
Regulatory Assets - Total	[1]	$ 305	$ 366
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	30 months	36 months
Deferred Lease Expenses [Member]
Regulatory Assets - Current	[1]	$ 0	$ 0
Regulatory Assets - Long -Term	[1]	58	54
Regulatory Assets - Total	[1]	$ 58	$ 54
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[1]	33 months	39 months
Minnesota Environmental Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]	$ 4	$ 4
Regulatory Assets - Long -Term	[2]	0	0
Regulatory Assets - Total	[2]	$ 4	$ 4
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]	12 months	12 months
South Dakota Transmission Cost Recovery Rider Accrued Revenues [Member]
Regulatory Assets - Current	[2]		$ 97
Regulatory Assets - Long -Term	[2]		0
Regulatory Assets - Total	[2]		$ 97
Regulatory Assets - Remaining Recovery/Refund Period (Month)	[2]		2 months

[1] Costs subject to recovery without a rate of return.

[2] Amount eligible for recovery under an alternative revenue program which includes an incentive or rate of return.