Asset Retirement Obligations and Accrued Environmental Costs - Textuals (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Environmental Liabilities [Line Items]
Accrued environmental costs	$ 180	$ 247
Additional Asset Retirement Obligations and Accrued Environmental Costs (Textual) [Abstract]
Expected expenditures for acquired environmental obligations, weighted-average discount factor rate	5.00%
Accrued environmental costs, undiscounted due within one year	$ 12
Accrued environmental costs, undiscounted due within second year	10
Accrued environmental costs, undiscounted due within three year	5
Accrued environmental costs, undiscounted due within fourth year	10
Accrued environmental costs, undiscounted due within fifth year	3
Accrued environmental costs, undiscounted due after fifth year	106
Cleanup Remediation Activities [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	105	183
Corporate and Other Environmental Liabilities [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	60	51
Federal Comprehensive Environmental Response Compensation and Liability Act or Similar State Laws [Member]
Environmental Liabilities [Line Items]
Accrued environmental costs	$ 15	$ 13