Nuclear Decommissioning Trust Fund (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Cash and cash equivalents year	Dec. 31, 2011 Cash and cash equivalents year	Sep. 30, 2012 U.S. government and federal agency obligations year	Dec. 31, 2011 U.S. government and federal agency obligations year	Sep. 30, 2012 Federal agency mortgage-backed securities year	Dec. 31, 2011 Federal agency mortgage-backed securities year	Sep. 30, 2012 Commercial mortgage-backed securities year	Dec. 31, 2011 Commercial mortgage-backed securities year	Sep. 30, 2012 Corporate debt securities year	Dec. 31, 2011 Corporate debt securities year	Sep. 30, 2012 Equity securities year	Dec. 31, 2011 Equity securities year	Sep. 30, 2012 Foreign government fixed income securities year	Dec. 31, 2011 Foreign government fixed income securities year
Nuclear decommissioning trust fund disclosure
Impairment charge on investment	$ 1	$ 3	$ 2	$ 495
Fair Value	469		469		424	4	2	33	43	63	63	6	7	72	54	286	251	5	4
Unrealized Gains	152		152		122	0	0	2	3	3	3	0	0	4	3	143	113	0	0
Unrealized Losses					2	0	0	0	0	0	0	0	0	0	1		1		0
Weighted-average maturities (in years)						0	0	11	10	23	23	29	28	11	10	0	0	6	8
Proceeds from sales of available-for-sale securities and the related realized gains and losses
Realized gains			8	4
Realized losses			5	3
Proceeds from sale of securities			$ 316	$ 294