Schedule II - Valuation and Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Uncollectible accounts	Dec. 31, 2010 Uncollectible accounts	Dec. 31, 2009 Uncollectible accounts	Dec. 31, 2011 Inventory valuation	Dec. 31, 2010 Inventory valuation	Dec. 31, 2009 Inventory valuation	Dec. 31, 2011 Fossil fuel plants dismantlement reserve	Dec. 31, 2010 Fossil fuel plants dismantlement reserve	Dec. 31, 2009 Fossil fuel plants dismantlement reserve	Dec. 31, 2011 Nuclear refueling outage reserve	Dec. 31, 2010 Nuclear refueling outage reserve	Dec. 31, 2009 Nuclear refueling outage reserve	Dec. 31, 2011 Deferred tax asset valuation allowance	Dec. 31, 2010 Deferred tax asset valuation allowance	Dec. 31, 2008 Deferred tax asset valuation allowance
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$ 35	$ 18	$ 18	$ 17	$ 14		$ 144	$ 143	$ 145	$ 15	$ 5	$ 14	$ 60	$ 55	$ 55
Additions charged to expenses	10	18	32	2	3	14	4	4	1	5	13	18	11	5
Charged to other accounts	1	24
Deductions	(19)	(25)	(32)	(2)				(3)	(3)		(3)	(27)
Balance at end of period	27	35	18	17	17	14	148	144	143	20	15	5	71	60	55
Parenthetical Data [Abstract]
Other non-customer allowance for doubtful receivables	$ (6)	$ 18
Number of coal-fired units to retire prior to the end of their estimated useful lives				11