Retirement Benefit Liabilities - Summary of Retirement Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021
Retirement Benefits [Abstract]
Retiree medical liability	$ 42	$ 42
Pension liability	131	141
Other	21	19
Subtotal	194	202
Less: current portion (included in compensation and benefits, Note 11)	(11)	(11)
Retirement benefits	$ 183	$ 191

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