Derivative Instruments and Hedging Activities - Location and Fair Values of Derivative Instruments and Hedging Activities (Detail) shares in Millions, $ in Millions	9 Months Ended
	Jun. 30, 2020 USD ($) T shares	Sep. 30, 2019 USD ($) T
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, amount of hedged ordinary shares | shares	1.4
Derivative assets	$ 70	$ 97
Derivative liabilities	2,627	$ 2,568
Equity swap
Derivative Instruments [Line Items]
Derivative, Amount of Hedged Item	$ 60
Copper [Member]
Derivative Instruments [Line Items]
Derivative, Nonmonetary Notional Amount | T	2,724	3,561
Aluminum [Member]
Derivative Instruments [Line Items]
Derivative, Nonmonetary Notional Amount | T	2,622	2,967
Other Current Liabilities | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative liabilities		$ 1
Designated as Hedging Instrument
Derivative Instruments [Line Items]
Derivative assets	$ 6	16
Derivative liabilities	2,611	2,568
Designated as Hedging Instrument | Other Current Assets | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative assets	5	16
Designated as Hedging Instrument | Other Current Assets | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative assets	1	0
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	9	23
Designated as Hedging Instrument | Other Current Liabilities | Commodity Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	0	1
Designated as Hedging Instrument | Long-term Debt | Foreign Currency Denominated Debt [Member]
Derivative Instruments [Line Items]
Derivative liabilities	2,602	2,544
Not Designated as Hedging Instrument
Derivative Instruments [Line Items]
Derivative assets	64	81
Derivative liabilities	16	0
Not Designated as Hedging Instrument | Other Current Assets | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative assets	15	19
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	49	62
Not Designated as Hedging Instrument | Other Current Liabilities | Foreign Currency Exchange Derivatives
Derivative Instruments [Line Items]
Derivative liabilities	16	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