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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2016 USD ($) tons-per-year | Mar. 31, 2015 USD ($) | Mar. 31, 2016 USD ($) tons-per-year | Mar. 31, 2015 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) tons-per-year | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($)
ASSETS |  |  |  |  |  |  |  |
Commodity contract derivatives | $ 4,000,000 |  | $ 4,000,000 |  |  | $ 1,000,000 |  |
Liabilities |  |  |  |  |  |  |  |
Commodity contract derivatives | $ 198,000,000 |  | $ 198,000,000 |  |  | $ 98,000,000 |  |
Maximum |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.7 |  | 0.7 |  |  | 1.0 |  |
Price per ton | $ 76.25 |  | $ 76.25 |  |  | $ 103.41 |  |
Liabilities |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.7 |  | 0.7 |  |  | 1.0 |  |
Price per ton | $ 76.25 |  | $ 76.25 |  |  | $ 103.41 |  |
Minimum |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.4 |  | 0.4 |  |  | 0.8 |  |
Price per ton | $ 9.45 |  | $ 9.45 |  |  | $ 10.64 |  |
Liabilities |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year | 0.4 |  | 0.4 |  |  | 0.8 |  |
Price per ton | $ 9.45 |  | $ 9.45 |  |  | $ 10.64 |  |
Fair Value, Inputs, Level 3 |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |
Commodity contract derivatives | 0 |  | 0 |  |  | 0 |  |
Liabilities |  |  |  |  |  |  |  |
Commodity contract derivatives | 187,000,000 |  | 187,000,000 |  |  | 98,000,000 |  |
Commodity Contract Derivatives |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |
Balance | (187,000,000) | $ (154,000,000) | (187,000,000) | $ (154,000,000) | $ (123,000,000) | $ (98,000,000) | $ (107,000,000) | $ (85,000,000)
Purchases | 0 | 0 | 0 | 0 |  |  |  |
Issuances | 0 | 0 | 0 | 0 |  |  |  |
Sales | 0 | 0 | 0 | 0 |  |  |  |
Settlements | 0 | 0 | 0 | 0 |  |  |  |
Net unrealized gains (losses) deferred as regulatory assets and liabilities | $ (64,000,000) | $ (47,000,000) | $ (89,000,000) | $ (69,000,000) |  |  |  |

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