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Income Taxes - Effective Income Tax Rate (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Federal Statutory Rate	21.00%	21.00%	21.00%	21.00%
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax	5.00%	5.00%	5.00%	5.00%
Production Tax Credits (PTCs)	(5.00%)	0.00%	(6.00%)	0.00%
Amortization of Excess Deferred Income Taxes	(1.60%)	(2.90%)	(2.00%)	(3.40%)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(0.20%)	(0.60%)	(0.30%)	(0.80%)
Excess Tax Deduction on Stock Awards	0.00%	0.30%	0.00%	(0.40%)
Allowance for Equity Funds Used During Construction	(0.10%)	(0.70%)	(0.10%)	(0.80%)
Other, Net	(0.80%)	(1.90%)	(0.70%)	(1.20%)
Effective Tax Rate	18.30%	20.20%	16.90%	19.40%

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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