Asset Impairment Expense (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				0 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended			9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Poland Wind Projects [Member]	Sep. 30, 2012 Poland Wind Projects [Member]	Sep. 30, 2013 Poland Wind Projects [Member]	Sep. 30, 2012 Poland Wind Projects [Member]	Sep. 30, 2013 Poland Wind Projects [Member] Minimum [Member]	Sep. 30, 2013 Poland Wind Projects [Member] Maximum [Member]	Jan. 09, 2013 Beaver Valley [Member] MW	Sep. 30, 2013 Beaver Valley [Member]	Sep. 30, 2012 Beaver Valley [Member]	Sep. 30, 2013 Beaver Valley [Member]	Sep. 30, 2012 Beaver Valley [Member]	Sep. 30, 2013 Itabo (San Lorenzo) [Member] MW		Sep. 30, 2012 Itabo (San Lorenzo) [Member]	Sep. 30, 2013 Itabo (San Lorenzo) [Member] MW		Sep. 30, 2012 Itabo (San Lorenzo) [Member]	Sep. 30, 2013 Wind Turbines And Projects [Member]	Sep. 30, 2012 Wind Turbines And Projects [Member]	Sep. 30, 2013 Wind Turbines And Projects [Member]	Sep. 30, 2012 Wind Turbines And Projects [Member]	Sep. 30, 2013 Kelanitissa [ Member]	Sep. 30, 2012 Kelanitissa [ Member]	Sep. 30, 2013 Kelanitissa [ Member]	Sep. 30, 2012 Kelanitissa [ Member]	Sep. 30, 2013 St. Patrick [Member]	Sep. 30, 2012 St. Patrick [Member]	Jun. 30, 2012 St. Patrick [Member]	Sep. 30, 2013 St. Patrick [Member]	Sep. 30, 2012 St. Patrick [Member]	Sep. 30, 2013 Other Impairment [Member]	Sep. 30, 2012 Other Impairment [Member]	Sep. 30, 2013 Other Impairment [Member]	Sep. 30, 2012 Other Impairment [Member]	Sep. 30, 2012 Wind turbines [Member]	Sep. 30, 2012 Wind Projects [Member]
Asset Impairment Expense [Line Items]
Asset impairment expense	$ 81	$ 43	$ 129	$ 71	$ 65	$ 0	$ 65	$ 0				$ 0	$ 0	$ 46	$ 0	$ 15		$ 0	$ 15		$ 0	$ 0	$ 36	$ 0	$ 40	$ 0	$ 5	$ 0	$ 17	$ 0	$ 0	$ 11	$ 0	$ 11	$ 1	$ 2	$ 3	$ 3	$ 20	$ 16
Ownership interests									61.00%	89.00%
Assets carrying amount					79		79									22	[1]		22	[1]							22		22			33							45
Assets, fair value					14		14									7	[1]		7	[1]							10		10			22
Fair Value Less Costs To Sell																																							25
Generation capacity (MW)											125					35			35
Lump sum payment received for termination of PPA											$ 60

[1] See Note 14 — Asset Impairment Expense for further information.