Summary of Significant Accounting Policies (Cash) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 463	$ 62
Book balances reclassified to accounts payable	33	69
Cash and cash equivalents	524	87
Short-term restricted cash	1	2
Total cash, cash equivalents, and restricted cash	525	89	$ 94	$ 70
SCE
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	438	38
Book balances reclassified to accounts payable	33	69
Cash and cash equivalents	484	55
Short-term restricted cash	1	1
Total cash, cash equivalents, and restricted cash	$ 485	$ 56	$ 30	$ 24