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Fair Value Measurements (Schedule of Equity Method Investments) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |
Revenue |  | $ 733 | $ 628 | $ 843
Net investment income | [1] | 1,699 | 1,834 | 1,824
Net income |  | 554 | 476 | 91
Total assets |  | 24,644 | 24,739 |
Total liabilities |  | 22,257 | 22,817 |
Non-controlling interests |  | 22 | 19 |
Alexander Forbes Group Holdings Limited |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Net investment income |  | $ 1,200 | $ 1,500 | $ 1,900
[1] | (a) Net investment income in 2018, 2017 and 2016 includes approximately $1.2 billion, $1.5 billion and $1.9 billion, respectively, related to Alexander Forbes, substantially all of which is credited to policy holders.

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