Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	$ 42	$ 62
Regulatory assets - noncurrent	426	438
Regulatory Liability, Current	18	31
Regulatory liabilities-noncurrent	1,379	1,288
Removal Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory liabilities-noncurrent	964	933
Deferred Income Tax Charge [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory liabilities-noncurrent	271	277
Environmental Restoration Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	1	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	37	0
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liability, Current	17	28
Regulatory liabilities-noncurrent	54	26
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	37	53
Regulatory assets - noncurrent	123	151
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets, Current	0	0
Regulatory assets - noncurrent	205	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	5	9
Regulatory assets - noncurrent	$ 55	$ 50