Retirement Benefit Liabilities - Components of Net Periodic Pension and Retiree Medical Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 10	$ 13	$ 31	$ 40
Assumed return on plan assets	(23)	(24)	(71)	(73)
Amortization of prior service benefit	0	0	0	0
Recognized actuarial loss	8	6	24	17
Total income	(5)	(5)	(16)	(16)
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	0	1	1	2
Assumed return on plan assets	0	0	0	0
Amortization of prior service benefit	(9)	(9)	(27)	(26)
Recognized actuarial loss	4	4	11	12
Total income	$ (5)	$ (4)	$ (15)	$ (12)