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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, 2018 USD ($) tons-per-year | Dec. 31, 2017 USD ($) | Sep. 30, 2018 USD ($) tons-per-year | Sep. 30, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity contract derivatives | $ 77,000,000 |  | $ 72,000,000 |
Commodity contract derivatives | $ 10,000,000 |  | $ 12,000,000 |
Maximum |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.8 |  | 0.8 |
Price per ton | $ 112.64 |  | $ 112.24 |
Minimum |  |  |  |
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 | $ 12.25 |  | $ 12.25 |
Fair Value, Inputs, Level 3 |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Commodity contract derivatives | 63,000,000 |  | 59,000,000 |
Commodity contract derivatives | 0 |  | 1,000,000 |
Commodity Contract Derivatives |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Balance | 63,000,000 | $ (73,000,000) | $ 58,000,000 | $ (67,000,000)
Net unrealized gains (losses) deferred as regulatory assets and liabilities | $ 5,000,000 | $ (6,000,000) |  |

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