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Financial Instruments - Narrative (Details) | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2023 USD ($) | Mar. 31, 2022 USD ($) | Mar. 31, 2023 USD ($) MMcf | Mar. 31, 2022 USD ($) | Apr. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Mar. 31, 2021 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |
Energy measure | MMcf |  |  | 4,123 |  |  |  |
Contract netting | $ 0 |  | $ 0 |  |  | $ 0 |
Cash collateral | 0 |  | 0 |  |  | $ 0 |
Net (gain) loss on settled interest rate agreements | (700,000) | $ 1,000,000 | (1,400,000) | $ 1,900,000 |  |  |
Net realized gains in AOCI | 93,100,000 |  | 93,100,000 |  |  |  |
Senior Notes |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  |  |  | $ 2,200,000,000
Forward Interest Rate Swap | Senior Notes |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt instrument, face amount | $ 150,000,000 |  | $ 150,000,000 |  |  |  |
Forward Interest Rate Swap | Senior Notes | Subsequent Event |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt instrument, face amount |  |  |  |  | $ 100,000,000 |  |
Gas Purchases | Not Designated As Hedges: | Commodity contracts |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Hedging percent | 32.00% |  | 32.00% |  |  |  |
Energy measure | MMcf |  |  | 17,700 |  |  |  |
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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