Retirement Benefit Liabilities - Summary of Retirement Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021
Retirement Benefits [Abstract]
Retiree medical liability	$ 41	$ 42
Pension liability	122	141
Other	19	19
Subtotal	182	202
Less: current portion (included in compensation and benefits, Note 11)	(11)	(11)
Retirement benefits	$ 171	$ 191

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