Contingencies - Summary of environmental reserves (Details) $ in Millions	3 Months Ended
	Dec. 31, 2017 USD ($)
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	$ 9
Payments and other	(1)
Accruals	1
Ending balance	9
Superfund Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	2
Payments and other	0
Accruals	0
Ending balance	2
Non-Superfund Sites
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance	7
Payments and other	(1)
Accruals	1
Ending balance	$ 7