Financial Instruments - Notional Amounts and Credit Risk Amounts Associated with Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Derivatives designated as accounting hedges | Foreign exchange contracts
Derivative [Line Items]
Derivative, notional amount	$ 48,122	$ 56,156
Derivative, credit risk amount	1,361	1,049
Derivatives designated as accounting hedges | Interest rate contracts
Derivative [Line Items]
Derivative, notional amount	35,250	33,000
Derivative, credit risk amount	12	218
Derivatives not designated as accounting hedges | Foreign exchange contracts
Derivative [Line Items]
Derivative, notional amount	61,179	69,774
Derivative, credit risk amount	$ 290	$ 363