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Fair Value Measurements Fair Value Measurements - Investments (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | Mar. 31, 2021 USD ($) | Mar. 31, 2020 USD ($) | Sep. 30, 2020 USD ($)
Investment Gains (Losses) |  |  |  |  |
Financing Receivable, after Allowance for Credit Loss | $ 107 |  | $ 107 |  | $ 93
Balance in NDT | $ 2,600 |  | $ 2,600 |  |
Period of time where the investor contributes capital to an investment in a private partnership - minimum |  |  | three |  |
Period of time where the investor contributes capital to an investment in a private partnership - maximum |  |  | four |  |
Minimum investment period |  |  | 10 years |  |
Number of readily available quoted exchange prices for the investments | 0 |  | 0 |  |
Investment funds | $ 3,723 |  | $ 3,723 |  | $ 3,198
LTDCP [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | 0 | $ 3 | (1) | $ 2 |
SERP |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (1) | 9 | (5) | 8 |
ART [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (28) | 135 | (120) | 98 |
NDT [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Unrealized gains (losses) on investments | (23) | $ 357 | 253 | $ 246 |
Equity Funds |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 220 |  | 220 |  |
Real Estate Funds |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 88 |  | 88 |  |
Credit [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 44 |  | 44 |  |
Private Equity Funds [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | 129 |  | 129 |  |
Private real estate funds [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
Fair value plan assets gross | 69 |  | 69 |  |
Private Credit [Member] |  |  |  |  |
Investment Gains (Losses) |  |  |  |  |
NDT unfunded commitments related to private equity and real estate | $ 22 |  | $ 22 |  |

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. This amount is not reduced by non-benefit liabilities. + References No definition available. + Details
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