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Financial Instruments - Narrative (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2022 USD ($) | Jun. 30, 2021 USD ($) | Jun. 30, 2022 USD ($) Bcf | Jun. 30, 2021 USD ($) | Sep. 30, 2021 USD ($)
Derivative [Line Items] |  |  |  |  |
Energy measure | Bcf |  |  | 5,819,000,000 |  |
Contract netting | $ 0 |  | $ 0 |  | $ 0
Cash collateral | 0 |  | 0 |  | $ 0
Net gain (loss) on settled interest rate agreements | (1,000,000) | $ (1,500,000) | (2,900,000) | $ (4,400,000) |
Net realized gain (loss) in AOCI | $ (59,500,000) |  | $ (59,500,000) |  |
Gas Purchases | Not Designated As Hedges: | Commodity contracts |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Hedging percent | 42.00% |  | 42.00% |  |
Energy measure | Bcf |  |  | 23.9 |  |
Minimum | Gas Purchases | Not Designated As Hedges: | Commodity contracts |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Hedging percent | 25.00% |  | 25.00% |  |
Maximum | Gas Purchases | Not Designated As Hedges: | Commodity contracts |  |  |  |  |
Derivative [Line Items] |  |  |  |  |
Hedging percent | 50.00% |  | 50.00% |  |

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