Pension and Other Postretirement Benefits - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	$ (5)	$ 114	$ (47)
Amortization of actuarial (loss) gain	(14)	(7)	(3)
Current year prior service cost	0	0	0
Total recognized in other comprehensive income (loss)	(19)	107	(50)
Total recognized in net periodic benefit cost and other comprehensive income (loss)	(49)	75	(85)
Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial loss (gain)	21	27	30
Amortization of actuarial (loss) gain	(2)	0	0
Current year prior service cost	0	1	0
Total recognized in other comprehensive income (loss)	19	28	30
Total recognized in net periodic benefit cost and other comprehensive income (loss)	$ 20	$ 27	$ 26