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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Jun. 30, 2022 USD ($) Units | Jun. 30, 2021 USD ($) | Jun. 30, 2022 USD ($) Units | Jun. 30, 2021 USD ($) | Sep. 30, 2021 USD ($)
Investment Gains (Losses) |  |  |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 120 |  | $ 120 |  | $ 94
Balance in NDT | $ 2,600 |  | $ 2,600 |  |
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 | $ 3,751 |  | $ 3,751 |  | 4,053
Affiliated Entity |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Investment funds | 438 |  | 438 |  | $ 573
LTDCP [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | 2 | $ (1) | 4 | $ (1) |
SERP |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | 9 | (3) | 14 | (8) |
ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | 139 | (48) | 147 | (168) |
NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | 331 | $ (107) | (305) | $ (361) |
Equity Funds |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 191 |  | 191 |  |
Real Estate Funds |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 128 |  | 128 |  |
Credit [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 50 |  | 50 |  |
Private Equity Funds [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 110 |  | 110 |  |
Private real estate funds [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Fair value plan assets gross | 78 |  | 78 |  |
Private Credit [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | $ 27 |  | $ 27 |  |

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