Summary of Significant Accounting Policies (Cash) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 360	$ 62
Book balances reclassified to accounts payable	59	69
Cash and cash equivalents	389	87
Short-term restricted cash	1	2
Total cash, cash equivalents, and restricted cash	390	89	$ 1,338	$ 70
SCE
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	3	38
Book balances reclassified to accounts payable	59	69
Cash and cash equivalents	25	55
Short-term restricted cash	1	1
Total cash, cash equivalents, and restricted cash	$ 26	$ 56	$ 452	$ 24