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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

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- Definition Greenhouse Gas Allowances + References No definition available. + Details
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