Retirement Benefit Liabilities - Summary of Retirement Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Retirement Benefits [Abstract]
Retiree medical liability	$ 51	$ 52
Pension liability	120	139
Other	17	17
Subtotal	188	208
Less: current portion (included in compensation and benefits, Note 11)	(12)	(12)
Retirement benefits	$ 176	$ 196

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