Shareholders' Equity - Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income, beginning balance	$ (2,548)	$ (2,548)	$ (2,283)	$ (1,909)
Other comprehensive loss before reclassifications, net of taxes		383	(338)	(427)
Amounts reclassified into income, net of taxes		77	74	52
Accumulated other comprehensive (loss) income, ending balance		(2,088)	(2,548)	(2,283)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income, beginning balance	(1,416)	(1,416)	(1,256)	(1,162)
Other comprehensive loss before reclassifications, net of taxes		124	(161)	(93)
Amounts reclassified into income, net of taxes		0	0	0
Accumulated other comprehensive (loss) income, ending balance		(1,292)	(1,416)	(1,256)
Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income, beginning balance	(1,040)	(1,040)	(1,005)	(729)
Other comprehensive loss before reclassifications, net of taxes	24	187	(101)	(325)
Amounts reclassified into income, net of taxes		68	66	49
Accumulated other comprehensive (loss) income, ending balance		(784)	(1,040)	(1,005)
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income, beginning balance	$ (91)	(91)	(23)	(17)
Other comprehensive loss before reclassifications, net of taxes		72	(76)	(9)
Amounts reclassified into income, net of taxes		9	8	3
Accumulated other comprehensive (loss) income, ending balance		$ (10)	$ (91)	$ (23)