Retirement Benefit Liabilities - Components of Net Periodic Pension and Retiree Medical Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 10	$ 14	$ 21	$ 27
Assumed return on plan assets	(24)	(25)	(48)	(49)
Amortization of prior service benefit	0	0	0	0
Recognized actuarial loss	8	5	16	11
Total income	(6)	(6)	(11)	(11)
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	1	0	1	1
Assumed return on plan assets	0	0	0	0
Amortization of prior service benefit	(9)	(8)	(18)	(17)
Recognized actuarial loss	3	4	7	8
Total income	$ (5)	$ (4)	$ (10)	$ (8)