Debt - Summary of Interest Costs and Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Debt Disclosure [Abstract]
Charged to operations	$ 706	$ 521	$ 388
Capitalized	42	32	30
Total interest costs	748	553	418
Interest paid, net of amounts capitalized	$ 674	$ 435	$ 392