Asset Retirement Obligations and Accrued Environmental Costs - Textuals (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Environmental Liabilities [Line Items]
Accrued environmental costs	$ 178	$ 180
Maximum number of years accrued environmental liabilities will be paid over	30 years
Expected expenditures for acquired environmental obligations, weighted-average discount factor rate	5.00%
Accrued environmental costs, undiscounted due 2019	$ 6
Accrued environmental costs, undiscounted due 2020	6
Accrued environmental costs, undiscounted due 2021	10
Accrued environmental costs, undiscounted due 2022	6
Accrued environmental costs, undiscounted due 2023	2
Accrued environmental costs, undiscounted due after 2023	109
Cleanup Remediation Activities [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	100	105
Corporate and Other Environmental Liabilities [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	67	60
Federal Comprehensive Environmental Response Compensation and Liability Act or Similar State Laws [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	11	$ 15
Acquired Environmental Obligations [Member]
Environmental Liabilities [Line Items]
Accrued Enviornmental Costs Discounted	$ 88