Investment Income, Net (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Fair Value Measurements and Investments [Abstract]
Interest and dividend income on cash and investments	$ 92	$ 75	$ 31
Gain on other investments	6	5	3
Investment securities, trading:
Unrealized gains (losses), net	6	3	(6)
Realized gains, net	2		2
Investment securities, available-for-sale:
Realized (losses) gains, net	(1)	3	21
Other-than-temporary impairment on investments		(4)	(5)
Investment income	$ 105	$ 82	$ 46