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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Dec. 31, 2021 USD ($) Units | Dec. 31, 2020 USD ($) | Sep. 30, 2021 USD ($)
Investment Gains (Losses) |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 99 |  | $ 94
Balance in NDT | $ 3,000 |  |
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 | $ 4,314 |  | 4,053
Affiliated Entity |  |  |
Investment Gains (Losses) |  |  |
Investment funds | 577 |  | $ 573
LTDCP [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | 0 | $ (1) |
SERP |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | (2) | (4) |
ART [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | (30) | (92) |
NDT [Member] |  |  |
Investment Gains (Losses) |  |  |
Unrealized gains (losses) on investments | (119) | $ (230) |
Equity Funds |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 205 |  |
Real Estate Funds |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 102 |  |
Credit [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 54 |  |
Private Equity Funds [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | 116 |  |
Private real estate funds [Member] |  |  |
Investment Gains (Losses) |  |  |
Fair value plan assets gross | 71 |  |
Private Credit [Member] |  |  |
Investment Gains (Losses) |  |  |
NDT unfunded commitments related to private equity and real estate | $ 29 |  |

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