Environmental (Details) $ in Millions	3 Months Ended			12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) sites	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Environmental Issue [Member]
Loss Contingencies [Line Items]
Increase in environmental expense	$ 8	$ 2	$ 8
Environmental Issue [Member]
Loss Contingencies [Line Items]
Number of alleged environmental contamination sites (in sites) | sites				205
Number of environmental superfund sites (in sites) | sites				19
Liability for third-party tort claims				$ 8	$ 9
Accrued obligations for environmental matters, current liabilities				$ 40	40
Estimated time frame for accrued cost distributions				10
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning balance				$ 317	342	$ 369
Accruals / changes in estimates				0	5	5
Payments				(19)	(30)	(32)
Ending balance				298	$ 317	$ 342
Environmental Issue [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Future costs to settle claims and associated liability				240
Environmental Issue [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Future costs to settle claims and associated liability				$ 390