Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Millions	Mar. 31, 2018 USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 3
Due in 1 to 5 Years	112
Due in 5 to 10 Years	209
Due after 10 Years	127
Total	451
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	9
Due in 5 to 10 Years	2
Due after 10 Years	66
Total	77
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	3
Due in 1 to 5 Years	87
Due in 5 to 10 Years	174
Due after 10 Years	57
Total	321
Non U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	16
Due in 5 to 10 Years	33
Due after 10 Years	4
Total	$ 53