Retirement Benefit Liabilities - Components of Net Periodic Pension and Retiree Medical Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ (8)	$ (10)	$ (17)	$ (21)
Assumed return on plan assets	24	24	48	48
Amortization of prior service benefit	0	0	0	0
Recognized actuarial loss	(8)	(8)	(16)	(16)
Total income	8	6	15	11
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	18	18
Recognized actuarial loss	(3)	(3)	(6)	(7)
Total income	$ 5	$ 5	$ 11	$ 10