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Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) MW | Dec. 31, 2017 USD ($)
NOV Consent Decree [Member] |  |
Environmental Matters [Line Items] |  |
Consent Decree Wind Energy Generation Addition (MW) | MW | 200 |
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) | $ 7 | $ 8
Expected Period to Incur Remediation Costs | 4 years |

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