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Contingencies (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2018 | Dec. 31, 1993
Loss Contingencies [Line Items] |  |  |  |  |  |
Malpractice Loss Contingency, Letters of Credit and Surety Bonds |  | $ 145.6 |  |  |  |
Long-term Purchase Commitment, Amount | $ 130.0 |  |  |  |  |
Proceeds from Insurance Settlement, Investing Activities | 120.0 | 120.0 | $ 0.0 | $ 0.0 |  |
Former Gain Contingency, Recognized in Current Period | $ 10.0 |  |  |  |  |
Loss Contingency, Damages Sought, Value |  | $ 1,200.0 |  |  |  |
Site Contingency, Names of Other Potentially Responsible Parties |  | 100 |  |  |  |
Litigation Settlement, Expense |  | $ 115.0 |  |  |  |
Loss Contingency, Range of Possible Loss, Maximum |  | 1,700.0 |  |  |  |
Loss Contingency, Range of Possible Loss, Portion Not Accrued |  | 500.0 |  |  |  |
Loss Contingency, Inestimable Loss |  | 1,100.0 |  |  |  |
Payments for Environmental Liabilities |  | 6.0 |  |  |  |
investment in Trojan |  |  |  |  |  | 87.00%
Class action damages sought |  | 260.0 |  |  |  |
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base |  | 514.0 |  |  |  |
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment |  |  |  |  | $ 640.0 |
Loss Contingency, Accrual, Current |  | $ 8.0 | $ 14.0 |  |  |

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