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Commitments, Guarantees and Contingencies - Environmental Matters (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Coal Combustion Residuals [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Expected Period for Costs of Compliance | 15 years |
Coal Combustion Residuals [Member] | Minimum [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | $ 65 |
Coal Combustion Residuals [Member] | Maximum [Member] | Minnesota Power [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | 120 |
Manufactured Gas Plant [Member] | SWL&P [Member] | Superior, WI [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance (Accrued) | $ 13 | $ 7

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