Summary of Significant Accounting Policies (Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 1,024	$ 41
Book balances reclassified to accounts payable	64	138
Restricted cash	41	18
SCE
Cash and Cash Equivalents Items [Line Items]
Money market funds	483	18
Book balances reclassified to accounts payable	63	$ 136
Restricted cash	$ 1