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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions | 3 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) tons-per-year | Dec. 31, 2016 USD ($) | Sep. 30, 2017 USD ($) tons-per-year | Sep. 30, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity contract derivatives | $ 16,000,000 |  | $ 10,000,000 |
Commodity contract derivatives | $ 93,000,000 |  | $ 70,000,000 |
Maximum |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.7 |  | 0.7 |
Price per ton | $ 112.81 |  | $ 112.23 |
Minimum |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.6 |  | 0.6 |
Price per ton | $ 12.15 |  | $ 11.40 |
Fair Value, Inputs, Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity contract derivatives | 9,000,000 |  | 2,000,000 |
Commodity contract derivatives | 82,000,000 |  | 69,000,000 |
Commodity Contract Derivatives |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Balance | (73,000,000) | $ (104,000,000) | $ (67,000,000) | $ (127,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities | $ (6,000,000) | $ 23,000,000 |  |

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- Definition Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 + Details
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