Retirement Benefit Liabilities - Components of net periodic pension and retiree medical expense (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 13	$ 13
Assumed return on plan assets	(24)	(23)
Amortization of prior service costs	0	0
Recognized actuarial loss	7	7
Total expense (income)	(4)	(3)
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	1	4
Assumed return on plan assets	0	0
Amortization of prior service costs	(9)	(1)
Recognized actuarial loss	4	4
Total expense (income)	$ (4)	$ 7