Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Millions	Dec. 31, 2017 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 5
Due in 1 to 5 Years	102
Due in 5 to 10 Years	205
Due after 10 Years	129
Total	441
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	2
Due in 5 to 10 Years	0
Due after 10 Years	67
Total	69
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	5
Due in 1 to 5 Years	85
Due in 5 to 10 Years	174
Due after 10 Years	58
Total	322
Non U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	15
Due in 5 to 10 Years	31
Due after 10 Years	4
Total	$ 50