Summary of Significant Accounting Policies (Emission Allowances) (Details) - SCE - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Other Current Assets
Significant Accounting Policies [Line Items]
GHG allowances	$ 113	$ 79
Other Current Liabilities
Significant Accounting Policies [Line Items]
GHG emission obligations	$ 95	$ 86