Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Millions	Jun. 30, 2018 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 7
Due in 1 to 5 Years	114
Due in 5 to 10 Years	204
Due after 10 Years	129
Total	454
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	4
Due in 5 to 10 Years	2
Due after 10 Years	69
Total	75
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	5
Due in 1 to 5 Years	90
Due in 5 to 10 Years	172
Due after 10 Years	56
Total	323
Non U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	2
Due in 1 to 5 Years	20
Due in 5 to 10 Years	30
Due after 10 Years	4
Total	$ 56