Retirement Benefit Liabilities - Components of Net Periodic Pension and Retiree Medical Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ (10)	$ (8)	$ (19)	$ (17)
Assumed return on plan assets	25	24	49	48
Amortization of prior service benefit	0	0	0	0
Recognized actuarial loss	(7)	(8)	(14)	(16)
Total income	8	8	16	15
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	(1)	(1)	(1)	(1)
Assumed return on plan assets	0	0	0	0
Amortization of prior service benefit	9	9	17	18
Recognized actuarial loss	(2)	(3)	(5)	(6)
Total income	$ 6	$ 5	$ 11	$ 11