Summary of Significant Accounting Policies (Restricted Cash) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 105	$ 1,091
Short-term restricted cash	1	40
Long-term restricted cash	0	(1)
Total cash, cash equivalents, and restricted cash	106	1,132	$ 169	$ 114
Restricted cash	27	41
Southern California Edison
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	15	515
Total cash, cash equivalents, and restricted cash	15	$ 515	$ 101	$ 40
SoCore Energy
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	18
Restricted cash	$ 26