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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2024 USD ($) Units | Mar. 31, 2023 USD ($) | Mar. 31, 2024 USD ($) Units | Mar. 31, 2023 USD ($) | Sep. 30, 2023 USD ($)
Investment Gains (Losses) |  |  |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 101 |  | $ 101 |  | $ 96
Period of time where the investor contributes capital to an investment in a private partnership - minimum | Units | 3 |  | 3 |  |
Period of time where the investor contributes capital to an investment in a private partnership - maximum | Units | 4 |  | 4 |  |
Minimum investment period | Units | 10 |  | 10 |  |
Number of readily available quoted exchange prices for the investments | 0 |  | 0 |  |
Investment funds | $ 4,655 |  | $ 4,655 |  | $ 4,123
NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Investment funds | 3,200 |  | 3,200 |  |
ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Investment funds | 1,400 |  | 1,400 |  |
LTDCP [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (1) | $ (1) | (2) | $ (1) |
SERP |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (2) | (4) | (9) | (7) |
ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (34) | (52) | (123) | (121) |
Debt and Equity Securities, Unrealized Gain (Loss) | 8 | 8 | 24 | 18 |
NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (54) | (105) | 251 | (241) |
Debt and Equity Securities, Unrealized Gain (Loss) | 24 | $ 34 | 70 | $ 60 |
Real Estate Funds | NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 112 |  | 112 |  |
Real Estate Funds | ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 55 |  | 55 |  |
Private Equity Funds [Member] | NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 323 |  | 323 |  |
Private Equity Funds [Member] | ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 120 |  | 120 |  |
Private Credit [Member] | NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 83 |  | 83 |  |
Private Credit [Member] | ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | $ 44 |  | $ 44 |  |

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