Fair Value Measurements (Schedule Of Activity For Debt And Equity Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Measurements [Abstract]
Proceeds from sales and maturities of nuclear decommissioning trust investments	$ 1,291	$ 1,295	$ 1,268
Gross realized gains on sales of securities held as available-for-sale	53	18	55
Gross realized losses on sales of securities held as available-for-sale	$ (11)	$ (26)	$ (37)