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Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) MW | Dec. 31, 2017 USD ($)
Environmental Restoration Costs [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance (Accrued) | $ 7.0 | $ 8.0
NOV Consent Decree [Member] |  |
Environmental Matters [Line Items] |  |
Consent Decree Wind Energy Generation Addition (MW) | MW | 200 |
Ozone NAAQS [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | $ 0.0 |
NO2 NAAQS [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | 0.0 |
Clean Water Act - Aquatic Organisms [Member] | Maximum [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | 15.0 |
Coal Combustion Residuals [Member] | Minimum [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | 65.0 |
Coal Combustion Residuals [Member] | Maximum [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Costs of Compliance | $ 100.0 |

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