Fair Value (Gains and Losses and Proceeds from the Sale of Securities by the Nuclear Decommissioning Trust Funds) (Details) (Nuclear Decommissioning Trust Fund [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Nuclear Decommissioning Trust Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Realized gains	$ 54	$ 83	$ 37
Realized losses	(33)	(41)	(31)
Proceeds from sales of securities	$ 1,146	$ 1,118	$ 759