Retirement Benefit Liabilities - Summary of Retirement Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Retiree medical liability	$ 63	$ 67
Pension liability	255	271
Other	14	14
Subtotal	332	352
Less: current portion (included in compensation and benefits, Note 15)	(17)	(16)
Retirement benefits	$ 315	$ 336

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