Derivative Instruments and Hedging Activities - Location and Fair Values of Derivative Instruments and Hedging Activities (Detail) shares in Thousands, $ in Millions	9 Months Ended
	Jun. 30, 2021 USD ($) T shares	Sep. 30, 2020 USD ($) T shares
Derivative Instruments [Line Items]
Hedge Percentage For Foreign Exchange Transactional Exposures Minimum	70.00%
Hedge Percentage For Foreign Exchange Transactional Exposures Maximum	90.00%
Derivative assets	$ 48	$ 87
Derivative liabilities	3,022	$ 3,020
Interest Rate Swaps
Derivative Instruments [Line Items]
Derivative, Notional Amount	$ 500
Copper [Member]
Derivative Instruments [Line Items]
Derivative, Nonmonetary Notional Amount | T	2,452	2,497
Aluminum [Member]
Derivative Instruments [Line Items]
Derivative, Nonmonetary Notional Amount | T	2,415	3,036
Equity swap
Derivative Instruments [Line Items]
Derivative, amount of hedged ordinary shares | shares	300	1,400
Derivative, Amount of Hedged Item	$ 23	$ 60
Designated as Hedging Instrument
Derivative Instruments [Line Items]
Derivative assets	15	12
Derivative liabilities	3,017	3,020
Designated as Hedging Instrument | Other Current Assets | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative assets	11	10
Designated as Hedging Instrument | Other Current Assets | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative assets	4	2
Designated as Hedging Instrument | Other Noncurrent Assets | Equity swap
Derivative Instruments [Line Items]
Derivative assets	0	0
Designated as Hedging Instrument | Other Current Liabilities | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	4	10
Designated as Hedging Instrument | Other Current Liabilities | Interest Rate Swaps
Derivative Instruments [Line Items]
Derivative liabilities	13	0
Designated as Hedging Instrument | Other Current Liabilities | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	1	0
Designated as Hedging Instrument | Long-term Debt | Foreign Currency Denominated Debt [Member]
Derivative Instruments [Line Items]
Derivative liabilities	2,999	3,010
Not Designated as Hedging Instrument
Derivative Instruments [Line Items]
Derivative assets	33	75
Derivative liabilities	5	0
Not Designated as Hedging Instrument | Other Current Assets | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative assets	10	17
Not Designated as Hedging Instrument | Other Current Assets | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative assets	0	0
Not Designated as Hedging Instrument | Other Noncurrent Assets | Equity swap
Derivative Instruments [Line Items]
Derivative assets	23	58
Not Designated as Hedging Instrument | Other Current Liabilities | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	5	0
Not Designated as Hedging Instrument | Other Current Liabilities | Interest Rate Swaps
Derivative Instruments [Line Items]
Derivative liabilities	0	0
Not Designated as Hedging Instrument | Other Current Liabilities | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	0	0
Not Designated as Hedging Instrument | Long-term Debt | Foreign Currency Denominated Debt [Member]
Derivative Instruments [Line Items]
Derivative liabilities	$ 0	$ 0