Fair Value Measurements (Schedule of Equity Method Investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Investment [Line Items]
Revenue	$ 719	$ 733	$ 628
Net investment income	959	1,699	1,834
Net income	482	554	476
Total assets	23,366	24,644
Total liabilities	21,013	22,257
Non-controlling interests	23	22
Alexander Forbes
Investment [Line Items]
Net investment income	$ 645	$ 1,200	$ 1,500