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Fair Value Measurements Fair Value Measurements - Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions | 12 Months Ended |
--- | --- | ---
Sep. 30, 2019 USD ($) tons-per-year | Sep. 30, 2018 USD ($) tons-per-year | Sep. 30, 2017 USD ($)
Fair Value Measurements |  |  |
Commodity contract derivatives, assets | $ 12,000,000 | $ 72,000,000 |
Commodity contract derivatives, liabilities | $ 53,000,000 | $ 12,000,000 |
Minimum |  |  |
Assets |  |  |
Fair value measurements tons per year | tons-per-year | 0.4 | 0.7 |
Price per ton | $ 12.10 | $ 12.25 |
Liabilities |  |  |
Fair value measurements tons per year | tons-per-year | 0.4 | 0.7 |
Price per ton | $ 12.10 | $ 12.25 |
Maximum |  |  |
Assets |  |  |
Fair value measurements tons per year | tons-per-year | 0.8 | 0.8 |
Price per ton | $ 94.51 | $ 112.24 |
Liabilities |  |  |
Fair value measurements tons per year | tons-per-year | 0.8 | 0.8 |
Price per ton | $ 94.51 | $ 112.24 |
Commodity Contract Derivatives |  |  |
Fair Value Measurements |  |  |
Balance at beginning/end of period | (4,000,000) | 58,000,000 | $ (67,000,000)
Change in net unrealized gains (losses) deferred as regulatory assets and liabilities | (62,000,000) | 125,000,000 |
Fair Value, Inputs, Level 3 |  |  |
Fair Value Measurements |  |  |
Commodity contract derivatives, assets | 5,000,000 | 59,000,000 |
Commodity contract derivatives, liabilities | $ 9,000,000 | $ 1,000,000 |

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