Financial Instruments - Derivative Instruments at Gross Fair Value (Details) - Level 2 - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Other current assets and other non-current assets | Foreign exchange contracts
Derivative assets:
Fair value of derivative assets	$ 1,235	$ 1,412
Other current assets and other non-current assets | Interest rate contracts
Derivative assets:
Fair value of derivative assets	0	218
Other current assets and other non-current assets | Derivatives designated as accounting hedges | Foreign exchange contracts
Derivative assets:
Fair value of derivative assets	850	1,049
Other current assets and other non-current assets | Derivatives designated as accounting hedges | Interest rate contracts
Derivative assets:
Fair value of derivative assets	0	218
Other current assets and other non-current assets | Derivatives not designated as accounting hedges | Foreign exchange contracts
Derivative assets:
Fair value of derivative assets	385	363
Other current assets and other non-current assets | Derivatives not designated as accounting hedges | Interest rate contracts
Derivative assets:
Fair value of derivative assets	0	0
Accrued expenses and other non-current liabilities | Foreign exchange contracts
Derivative liabilities:
Fair value of derivative liabilities	578	1,260
Accrued expenses and other non-current liabilities | Interest rate contracts
Derivative liabilities:
Fair value of derivative liabilities	1,271	303
Accrued expenses and other non-current liabilities | Derivatives designated as accounting hedges | Foreign exchange contracts
Derivative liabilities:
Fair value of derivative liabilities	357	759
Accrued expenses and other non-current liabilities | Derivatives designated as accounting hedges | Interest rate contracts
Derivative liabilities:
Fair value of derivative liabilities	1,271	303
Accrued expenses and other non-current liabilities | Derivatives not designated as accounting hedges | Foreign exchange contracts
Derivative liabilities:
Fair value of derivative liabilities	221	501
Accrued expenses and other non-current liabilities | Derivatives not designated as accounting hedges | Interest rate contracts
Derivative liabilities:
Fair value of derivative liabilities	$ 0	$ 0