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Income Taxes - Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal Statutory Rate	21.00%	21.00%	21.00%
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax	4.00%	3.40%	5.30%
Differences Reversing in Excess of Federal Rates	(3.60%)	(3.20%)	(3.60%)
Production Tax Credits (PTCs)	(1.10%)	0.00%	(3.20%)
North Dakota Wind Tax Credit Amortization, Net of Federal Taxes	(0.90%)	(1.00%)	(1.10%)
Allowance for Equity Funds Used During Construction	(0.70%)	(0.50%)	(0.40%)
Corporate-Owned Life Insurance	(0.60%)	(0.70%)	0.00%
Excess Tax Deduction on Stock Awards	(0.40%)	(0.70%)	(0.70%)
Other, Net	(0.30%)	(1.60%)	(2.30%)
Effective Tax Rate	17.40%	16.70%	15.00%

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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
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for dividend paid to employee stock ownership plan.

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