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Hedging Activities - Narrative (Details) £ in Millions, $ in Millions, € in Billions | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2023 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($) | Mar. 31, 2023 USD ($) | Jun. 15, 2023 USD ($) | Dec. 31, 2022 GBP (£) | Sep. 30, 2022 EUR (€)
Derivative [Line Items] |  |  |  |  |  |  |
Debt outstanding |  | $ 5,584 |  | $ 5,594 |  |  |
Disposed of by sale | U.K. Disposal Group (Disposal Group) |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Reclassification of gains from AOCI |  |  | $ 26 |  |  |  |
Fair Value Hedges | USD | Floating interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, notional amount |  | $ 1,250 |  | 1,250 |  |  |
Cash Flow Hedges | Floating interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, notional amount, entered into during period | $ 50 |  |  | 450 |  |  |
Derivative, notional amount |  |  |  |  | $ 500 |  |
Derivative termination proceeds, amortization period (in years) |  | 10 years |  |  |  |  |
Cash Flow Hedges | USD | Fixed interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, notional amount |  | $ 0 |  | $ 450 |  |  |
Currency swap | Fair Value Hedges | EUR | Derivatives designated for hedge accounting |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, notional amount | € |  |  |  |  |  |  | € 1.1
British Pound Sterling Denominated Notes | Term Loan |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Debt outstanding | £ |  |  |  |  |  | £ 450 |

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