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Income Taxes - Effective Income Tax Rate (Details) | 3 Months Ended | 9 Months Ended
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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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