Debt - Summary of Interest Costs and Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Debt Disclosure [Abstract]
Charged to operations	$ 639	$ 706	$ 521
Capitalized	44	42	32
Total interest costs	683	748	553
Interest paid, net of amounts capitalized	$ 658	$ 674	$ 435