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