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Investment Income, Net (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Fair Value Measurements and Investments [Abstract]      
Interest and dividend income on cash and investments $ 75 $ 31 $ 25
Gain on other investments 5 3 8
Investment securities, trading:      
Unrealized gains (losses), net 3 (6) (2)
Realized gains (losses), net   2 6
Investment securities, available-for-sale:      
Realized gains, net 3 21 1
Other-than-temporary impairment on investments (4) (5) (3)
Investment income $ 82 $ 46 $ 35