Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended							6 Months Ended	12 Months Ended				1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2010 Certain Non-Core Generation Facilities [Member]	Dec. 31, 2011 Certain Non-Core Generation Facilities [Member]		Dec. 31, 2010 Certain Non-Core Generation Facilities [Member]		Dec. 31, 2009 Certain Non-Core Generation Facilities [Member]		Mar. 31, 2011 Certain Non-Core Generation Facilities [Member]	Jun. 30, 2009 Long Island Generation Business [Member]	Dec. 31, 2011 Long Island Generation Business [Member]	Dec. 31, 2010 Long Island Generation Business [Member]	Dec. 31, 2009 Long Island Generation Business [Member]		Dec. 31, 2010 Maine Hydroelectric Generation Business [Member]	Dec. 31, 2010 Maine Hydroelectric Generation Business [Member]		Dec. 31, 2009 Maine Hydroelectric Generation Business [Member]		Dec. 31, 2011 Western Kentucky Energy Corp Lease [Member]	Dec. 31, 2010 Western Kentucky Energy Corp Lease [Member]
Discontinued Operations (Numeric) [Abstract]
Proceeds from the sale of certain non-core generation facilities	$ 381	$ 0	$ 0
Carrying amount of net assets held for sale				473
Estimated fair value of assets held for sale				377
Initial pre-tax impairment charge				96								52
Initial after-tax impairment charge				58
After-tax impairment charge												34
Subsequent pre-tax impairment charge															10
Subsequent after-tax impairment charge															3
Goodwill allocated to an asset or group of assets and written off, pre-tax				5
Goodwill allocated to an asset or group of assets and written off, after-tax				4
PP&E removed from Balance Sheet as a result of sale											357
Equity investment removed from Balance Sheet as part of sale											14
Agreed upon sales price for disposal of a business, plant, facility or unit															135
Proceeds from the sale of Long Island generation business	0	124	0
Proceeds from the sale of the remaining Maine hydroelectric facilities																		24
Number of fully owned hydroelectric facilities sold																	3	3		5
Proceeds from the sale of the majority of the Maine hydroelectric business	0	38	81
Gain (loss) on the sale of discontinued operations																	14	11		38
Gain (loss) on the sale of discontinued operations, net of tax																	8	7		22
Ownership percentage in a Maine hydroelectric facility sold																				50.00%
Contingent consideration received																	14
Additional tax expense recorded due to adjustments to tax basis			24
Term, in years, of lease to operate generating facilities																						25
Number of generating plants under lease																						9
Maximum potential liability to specific aluminum smelter due to lease termination																						82
Recorded liability to another aluminum smelter																							4
Discontinued Operations (Details) [Abstract]
Operating revenues					19		113		106				0	0	24			0		5
Operating expenses					11		156	[1]	42				0	0	73	[2]		(25)	[3]	(34)	[3]
Operating income					8		(43)		64				0	0	(49)			25		39
Other income (expense) - net					0		2		2									0		3
Interest expense					3	[4]	11	[4]	9	[4]			0	0	4	[5]		0		1	[6]
Income before income taxes					5		(52)		57				0	0	(53)			25		41
Income tax expense	2	(6)	46		3		(18)		24				0	0	(20)			10		17
Income (Loss) from Discontinued Operations					$ 2		$ (34)		$ 33				$ 0	$ 0	$ (33)			$ 15		$ 24

[1] 2010 includes the impairments to the carrying value of the non-core generation facilities and the write-off of allocated goodwill.

[2] Includes impairment charges.

[3] Includes the gains recorded on the sales.

[4] Represents allocated interest expense based upon debt attributable to the generation facilities sold.

[5] Represents allocated interest expense based upon debt attributable to the Long Island generation business sold.

[6] Represents allocated interest expense based upon debt attributable to the Maine hydroelectric generation business sold.