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, 2021	Sep. 30, 2020	Sep. 30, 2019
U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	$ (143)	$ (5)	$ 114
Amortization of actuarial gain (loss)	(3)	(14)	(7)
Current year prior service cost	0	0	0
Total recognized in other comprehensive (income) loss	(146)	(19)	107
Total recognized in net periodic benefit cost and other comprehensive (income) loss	(189)	(49)	75
Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Current year actuarial (gain) loss	(45)	21	27
Amortization of actuarial gain (loss)	(6)	(2)	0
Current year prior service cost	0	0	1
Total recognized in other comprehensive (income) loss	(51)	19	28
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$ (48)	$ 20	$ 27

Amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss.

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Amount, before tax, after reclassification adjustment, of increase (decrease) in accumulated other comprehensive income from gain (loss) of defined benefit plan.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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