Retirement Benefit Liabilities - Summary of retirement benefits (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Retirement Benefits [Abstract]
Retiree medical liability	$ 84	$ 86
Pension liability	176	182
Other	14	13
Subtotal	274	281
Less: current portion (included in compensation and benefits, Note 16)	(20)	(19)
Retirement benefits	$ 254	$ 262

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