Summary of Significant Accounting Policies (Cash) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	$ 47	$ 62
Book balances reclassified to accounts payable	61	69
Cash and cash equivalents	84	87
Short-term restricted cash	1	2
Total cash, cash equivalents, and restricted cash	85	89	$ 526	$ 70
SCE
Restricted Cash and Cash Equivalents Items [Line Items]
Money market funds	20	38
Book balances reclassified to accounts payable	61	69
Cash and cash equivalents	51	55
Short-term restricted cash		1
Total cash, cash equivalents, and restricted cash	$ 51	$ 56	$ 254	$ 24