Debt - Summary of Interest Costs and Payments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Debt Disclosure [Abstract]
Charged to operations	$ 521	$ 388	$ 371
Capitalized	32	30	30
Total interest costs	553	418	401
Interest paid, net of amounts capitalized	$ 435	$ 392	$ 313