Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets		$ 119	$ 88
Total Non Current Regulatory Assets		542	655
TOTAL REGULATORY ASSETS		661	743
Total Current Regulatory Liabilities		17	41
Total Non Current Regulatory Liabilities		1,079	802
TOTAL REGULATORY LIABILITIES		1,096	843
Efficiency Program Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities		10	14
Deferred Income Tax Charge [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Liabilities		243	2
Asset Retirement Obligation Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Liabilities		830	795
Other Regulatory Liabilities
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities		7	27
Total Non Current Regulatory Liabilities		6	5
El Salvador Tariff Recoveries
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets		59	54
Pension Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Assets	[1]	298	316
Income Taxes Recoverable From Customers
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Assets	[1]	0	87
Deferred Midwest Independent Service Operator Costs
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Assets		102	114
Environmental Restoration Costs [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non Current Regulatory Assets		48	41
Other Regulatory Assets
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets		60	34
Total Non Current Regulatory Assets		$ 94	$ 97

[1] Past expenditures on which the Company earns a rate of return.