Debt - Summary of Interest Costs and Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Debt Disclosure [Abstract]
Charged to operations	$ 469	$ 528	$ 639
Capitalized	44	43	44
Total interest costs	512	571	683
Interest paid, net of amounts capitalized	$ 474	$ 515	$ 658