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Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current $ 56 $ 62
Regulatory assets - noncurrent 441 438
Regulatory Liability, Current 22 31
Regulatory liabilities-noncurrent 1,348 1,288
Removal Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 0 0
Regulatory liabilities-noncurrent 955 933
Deferred Income Tax Charge [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 0 0
Regulatory liabilities-noncurrent 272 277
Environmental Restoration Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 2 3
Regulatory liabilities-noncurrent 0 0
Asset Retirement Obligation Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 0 0
Regulatory liabilities-noncurrent 53 52
Revenue Subject to Refund [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities-noncurrent 25  
Other Regulatory Assets (Liabilities) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Liability, Current 20 28
Regulatory liabilities-noncurrent 43 26
Deferred Derivative Gain (Loss) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 48 53
Regulatory assets - noncurrent 135 151
Pension and Other Postretirement Plans Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 0 0
Regulatory assets - noncurrent 209 218
Loss on Reacquired Debt [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 0 0
Regulatory assets - noncurrent 17 19
Environmental Restoration Costs [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 0 0
Regulatory assets - noncurrent 26 0
Other Regulatory Assets (Liabilities) [Member]    
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets, Current 8 9
Regulatory assets - noncurrent $ 54 $ 50