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Summary of Significant Accounting Policies (Emission Allowances) (Details) - SCE - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Other Current Assets    
Significant Accounting Policies [Line Items]    
GHG allowances $ 127 $ 113
Other Current Liabilities    
Significant Accounting Policies [Line Items]    
GHG emission obligations $ 129 $ 95