Commitments And Contingencies (Third-Party Power Purchase Agreements) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Qualifying Facilities [Member]
Third-Party Power Purchase Agreements [Line Items]
Long term contract for purchase commitments, date of contract expiration, beginning date	2018
Long term contract for purchase commitments, date of contract expiration, ending date	2028
Present value of fixed capacity payments, portion classified as current liabilities	$ 11	$ 17
Present value of fixed capacity payments, portion classified as noncurrent liabilities	7	18
Capitalized asset for fixed capacity payments for corresponding assets	18	35
Capitalized asset for fixed capacity payments, accumulated amortization	$ 143	148
Renewable Energy [Member]
Third-Party Power Purchase Agreements [Line Items]
Long term contract for purchase commitments, date of contract expiration, beginning date	2018
Long term contract for purchase commitments, date of contract expiration, ending date	2043
Power Purchases and Electric Capacity [Member]
Third-Party Power Purchase Agreements [Line Items]
Costs Of Power Purchase	$ 3,300	$ 3,500	$ 3,500
Conventional Energy [Member]
Third-Party Power Purchase Agreements [Line Items]
Long term contract for purchase commitments, date of contract expiration, beginning date	2018
Long term contract for purchase commitments, date of contract expiration, ending date	2033