Summary of Significant Accounting Policies (Cash, Cash Equivalents and Restricted Cash) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 62	$ 31
Book balances reclassified to accounts payable	69	75
Cash and cash equivalents	87	68
Short-term restricted cash	2	2
Total cash, cash equivalents, and restricted cash	89	70	$ 152	$ 1,132
SCE
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	38	0
Book balances reclassified to accounts payable	69	74
Cash and cash equivalents	55	24
Short-term restricted cash	1	0
Total cash, cash equivalents, and restricted cash	$ 56	$ 24	$ 22	$ 515