Retirement Benefits - Components of Expense (Income) for Pension Benefits and Other Retirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Components of Expense (Income)
Service cost	$ 3	$ 3
Interest cost	30	28
Expected return on plan assets	(60)	(60)
Amortization:
Prior service credit	0	0
Net actuarial loss	20	23
Net benefit expense (income)	(7)	(6)
Other Retirement Benefits
Components of Expense (Income)
Service cost	0	0
Interest cost	1	1
Expected return on plan assets	0	0
Amortization:
Prior service credit	0	0
Net actuarial loss	2	2
Net benefit expense (income)	$ 3	$ 3