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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) tons-per-year in Billions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2015 USD ($) tons-per-year | Jun. 30, 2014 USD ($) | Jun. 30, 2015 USD ($) tons-per-year | Jun. 30, 2014 USD ($) | Sep. 30, 2014 USD ($) Contracts tons-per-year | Mar. 31, 2015 USD ($) | Mar. 31, 2014 USD ($) | Sep. 30, 2013 USD ($)
ASSETS |  |  |  |  |  |  |  |  |
Commodity contract derivatives |  | $ 3,000,000 |  | $ 3,000,000 |  | $ 1,000,000 |  |  |
Liabilities |  |  |  |  |  |  |  |  |
Commodity contract derivatives |  | 128,000,000 |  | 128,000,000 |  | $ 97,000,000 |  |  |
Number of contracts | Contracts |  |  |  |  |  | 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |
Amount of gain (loss) recognized in income |  | $ 0 |  | $ 0 |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |
Fair value inputs, counterparty credit risk | [1] |  |  |  |  | 5.00% |  |  |
Fair value measurements tons per year | tons-per-year |  | 1.0 |  | 1.0 |  | 1.1 |  |  |
Price per ton |  | $ 103.41 |  | $ 103.41 |  | $ 67.07 |  |  |
Liabilities |  |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year |  | 1.0 |  | 1.0 |  | 1.1 |  |  |
Price per ton |  | $ 103.41 |  | $ 103.41 |  | $ 67.07 |  |  |
Minimum |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |
Fair value inputs, counterparty credit risk | [1] |  |  |  |  | 2.00% |  |  |
Fair value measurements tons per year | tons-per-year |  | 0.8 |  | 0.8 |  | 1.0 |  |  |
Price per ton |  | $ 10.08 |  | $ 10.08 |  | $ 11.24 |  |  |
Liabilities |  |  |  |  |  |  |  |  |
Fair value measurements tons per year | tons-per-year |  | 0.8 |  | 0.8 |  | 1.0 |  |  |
Price per ton |  | $ 10.08 |  | $ 10.08 |  | $ 11.24 |  |  |
Fair Value, Inputs, Level 3 |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |
Commodity contract derivatives |  | 3,000,000 |  | 3,000,000 |  | 1,000,000 |  |  |
Liabilities |  |  |  |  |  |  |  |  |
Commodity contract derivatives |  | 128,000,000 |  | 128,000,000 |  | 97,000,000 |  |  |
Private Partnerships |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |
Balance |  | 238,000,000 | $ 188,000,000 | 238,000,000 | $ 188,000,000 | 214,000,000 | $ 223,000,000 | $ 180,000,000 | $ 159,000,000
Purchases |  | 11,000,000 | 7,000,000 | 21,000,000 | 23,000,000 |  |  |  |
Issuances |  | 0 | 0 | 0 | 0 |  |  |  |
Sales |  | (5,000,000) | (6,000,000) | (12,000,000) | (7,000,000) |  |  |  |
Settlements |  | 0 | 0 | 0 | 0 |  |  |  |
Net unrealized gains (losses) deferred as regulatory assets and liabilities |  | 9,000,000 | 7,000,000 | 15,000,000 | 13,000,000 |  |  |  |
Commodity Contract Derivatives |  |  |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |
Balance |  | (125,000,000) | (146,000,000) | (125,000,000) | (146,000,000) | $ (96,000,000) | $ (149,000,000) | $ (131,000,000) | $ (140,000,000)
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 |  | $ 24,000,000 | $ (15,000,000) | $ (29,000,000) | $ (6,000,000) |  |  |  |
[1] | Applies to two contracts.

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