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Contingencies (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 1997 | Dec. 31, 2016 | Sep. 30, 2008 | Dec. 31, 1993
Loss Contingencies [Line Items] |  |  |  |  |  |
Malpractice Loss Contingency, Letters of Credit and Surety Bonds | $ 145.6 |  |  |  |  |
Long-term Purchase Commitment, Amount |  | $ 200.0 |  |  |  |
Loss Contingency, Damages Sought, Value |  | 1,200.0 |  |  |  |
Liability for Title Claims and Claims Adjustment Expense |  | $ 8.0 |  |  |  |
Site Contingency, Names of Other Potentially Responsible Parties |  | 100 | 69 |  |  |
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 |  |  |  |
investment in Trojan |  |  |  |  |  | 87.00%
Class action damages sought |  | 260.0 |  |  |  |
Refund to customers for Trojan Investment including interest |  |  |  |  | $ 33.0 |
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 | 637.0 |  |  |  |  |
Loss Contingency, Accrual, Current |  | $ 14.0 |  | $ 3.0 |  |
Minimum [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Loss Contingency, Damages Sought, Value | 44.0 |  |  |  |  |
Maximum [Member] |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |
Loss Contingency, Damages Sought, Value | $ 117.0 |  |  |  |  |

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- Definition The portion of the Company's remaining investment in its Trojan plant that the OPUC allowed the Company to recover. + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling asserted and unasserted claims estimated to have been incurred by the insureds on or before the balance sheet date pertaining to the existence of a lien, encumbrance, defect or other matter purportedly affecting the title to the interest as insured. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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