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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2024 USD ($) Units | Dec. 31, 2023 USD ($) | Sep. 30, 2024 USD ($)
Investment Gains (Losses) |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 86 |  | $ 99
Period of time where the investor contributes capital to an investment in a private partnership - minimum | Units | 3 |  |
Period of time where the investor contributes capital to an investment in a private partnership - maximum | Units | 4 |  |
Minimum investment period | Units | 10 |  |
Number of readily available quoted exchange prices for the investments | 0 |  |
Investment funds | $ 4,919 |  | $ 4,968
NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
Investment funds | 3,300 |  |
ART [Member] |  |  |
Investment Gains (Losses) |  |  |
Investment funds | 1,500 |  |
LTDCP [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | 1 | $ (1) |
SERP |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | 6 | (7) |
ART [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | 20 | (89) |
Debt and Equity Securities, Unrealized Gain (Loss) | (11) | 18 |
NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | 75 | (197) |
Debt and Equity Securities, Unrealized Gain (Loss) | (43) | $ 61 |
Real Estate Funds | NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 129 |  |
Real Estate Funds | ART [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 68 |  |
Private Equity Funds [Member] | NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 380 |  |
Private Equity Funds [Member] | ART [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 146 |  |
Private Credit [Member] | NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 89 |  |
Private Credit [Member] | ART [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | $ 47 |  |

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