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Hedging Activities - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Jun. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Oct. 31, 2024 CAD ($) | Sep. 30, 2024 CAD ($) | Sep. 30, 2024 USD ($) | Jun. 30, 2024 CAD ($) | Mar. 31, 2024 CAD ($) | Jun. 15, 2023 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), cash flows hedge, gain (loss) reclassification, before tax | $ 0 | $ 0 |  | $ 0 | $ 0 |  |  |  |  |  |  |
Net Investment Hedging | CAD | Currency swap | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative, notional amount |  |  |  |  |  |  |  | $ 2,500 |  | $ 2,500 | $ 1,500 |
Derivative notional amount, terminated |  |  |  |  |  |  |  |  |  | $ 1,500 |  |
Net Investment Hedging | CAD | Currency swap | Derivatives designated for hedge accounting | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative, notional amount |  |  |  |  |  |  | $ 1,000 |  |  |  |  |
Fair Value Hedges | Floating interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative, notional amount |  |  |  |  |  | $ 1,300,000,000 |  |  | $ 1,300,000,000 |  |  |
Derivative notional amount, terminated |  |  |  |  |  |  |  |  | $ 500,000,000 |  |  |
Cash Flow Hedges | Currency swap | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), cash flows hedge, gain (loss) reclassification, before tax | $ (14,000,000) | $ 33,000,000 |  | $ (14,000,000) | $ 27,000,000 |  |  |  |  |  |  |
Cash Flow Hedges | Floating interest rate swaps | Derivatives designated for hedge accounting |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative, notional amount |  |  |  |  |  |  |  |  |  |  |  | $ 500,000,000
Derivative, notional amount, entered into during period |  |  | $ 50,000,000 |  |  | $ 450,000,000 |  |  |  |  |  |
Derivative termination proceeds, amortization period (in years) |  |  |  | 10 years |  |  |  |  |  |  |  |

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