Retirement Benefit Liabilities - Components of net periodic pension and retiree medical expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 13	$ 16	$ 39	$ 49
Assumed return on plan assets	(24)	(24)	(71)	(74)
Amortization of prior service costs	0	0	0	0
Recognized actuarial loss	8	6	23	18
Total expense (income)	(3)	(2)	(9)	(7)
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	4	4	11	13
Assumed return on plan assets	0	0	0	0
Amortization of prior service costs	(1)	0	(2)	0
Recognized actuarial loss	4	3	11	9
Total expense (income)	$ 7	$ 7	$ 20	$ 22