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Contingencies (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 1997 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2008 | Dec. 31, 1993
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Malpractice Loss Contingency, Letters of Credit and Surety Bonds | $ 145.6 | $ 145.6 |  |  |  |  |
Long-term Purchase Commitment, Amount | 200.0 | 200.0 |  |  |  |  |
Loss Contingency, Damages Sought, Value |  | 1,200.0 |  |  |  |  |
Construction in Progress, Gross | 634.0 | 634.0 |  |  |  |  |
Total capital spending on new generation asset | 640.0 | 640.0 |  |  |  |  |
Liability for Title Claims and Claims Adjustment Expense | 17.0 | 17.0 |  |  |  |  |
Loss Contingency Accrual | 0.0 | $ 0.0 |  |  |  |  |
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 | 1,700.0 |  |  |  |  |
Loss Contingency, Range of Possible Loss, Portion Not Accrued | 500.0 | 500.0 |  |  |  |  |
Loss Contingency, Inestimable Loss | 1,050.0 | 1,050.0 |  |  |  |  |
investment in Trojan |  |  |  |  |  |  | 87.00%
Refund to customers for Trojan Investment including interest |  |  |  |  |  | $ 33.0 |
Class action damages sought |  |  |  |  | $ 260.0 |  |
Litigation Settlement, Amount | $ 1.0 | $ 79.0 |  |  |  |  |
Jointly Owned Utility Plant, Proportionate Ownership Share of Colstrip | 20.00% | 20.00% |  |  |  |  |
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base |  |  |  | $ 514.0 |  |  |

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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 proportionate share of the reporting entity's interest (percent) in Colstrip, a jointly owned utility plant. + References No definition available. + Details
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- Definition Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + 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 -Glossary Liability for Claim Adjustment Expenses -URI http://asc.fasb.org/extlink&oid=6516805 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 Reference 3: 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 + Details
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