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Income Taxes - Effective Income Tax Rate (Details)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Federal Statutory Rate	21.00%	21.00%
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax	5.00%	5.00%
Production Tax Credits (PTCs)	(7.60%)	0.00%
Amortization of Excess Deferred Income Taxes	(2.90%)	(4.10%)
Corporate-Owned Life Insurance	(0.50%)	0.70%
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(0.40%)	(0.90%)
Excess Tax Deduction on Stock Awards	(0.10%)	(1.30%)
Allowance for Equity Funds Used During Construction	0.00%	(1.00%)
Other, Net	0.30%	(0.50%)
Effective Tax Rate	14.80%	18.90%

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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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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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