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.