Debt - Summary of Interest Costs and Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Disclosure [Abstract]
Charged to operations	$ 528	$ 639	$ 706
Capitalized	43	44	42
Total interest costs	571	683	748
Interest paid, net of amounts capitalized	$ 515	$ 658	$ 674