Retirement Benefit Liabilities - Components of net periodic pension and retiree medical expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 13	$ 27	$ 26	$ 33
Assumed return on plan assets	(24)	(25)	(47)	(50)
Amortization of prior service costs	0	0	0	0
Recognized actuarial loss	8	6	15	12
Total expense (income)	(3)	8	(6)	(5)
Retiree Medical
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	3	5	7	9
Assumed return on plan assets	0	0	0	0
Amortization of prior service costs	0	0	1	0
Recognized actuarial loss	3	3	7	6
Total expense (income)	$ 6	$ 8	$ 13	$ 15