Business Acquisitions and Dispositions (Details) In Millions, except Share data, unless otherwise specified	0 Months Ended				0 Months Ended				0 Months Ended	9 Months Ended
	Jul. 19, 2012 USD ($)	Jul. 17, 2012 USD ($) MW	Jul. 17, 2012 EUR (€)	Jan. 18, 2012 NRG Solar AC Holdings LLC [Member]	Jan. 18, 2012 Sale of Interest in Agua Caliente [Member] USD ($)	Jul. 17, 2012 Kraftwerke Schkopau [Member]	Jul. 17, 2012 Kraftwerke Schkopau Betriebsgesllshaft mbH [Member]	Jul. 17, 2012 Schkopau [Member] MW	Jul. 22, 2012 GenOn Energy [Member]	Sep. 30, 2012 GenOn Energy [Member]	Jul. 20, 2012 GenOn Energy [Member] MW
Business Acquisition [Line Items]
Business Acquisition, Contingent Consideration, Shares Issuable									0.1216
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares										94,000,000
Business Acquisition, Purchase Price, Percentage of Outstanding Common Shares										29.00%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners				49.00%	49.00%
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Changes, Sale of Interest by Parent					$ 122
Investment Owned, Percent of Net Assets								100.00%
Equity Method Investment, Ownership Percentage						41.90%	44.40%
Power Generation, Participation, Megawatts		400	400
Generation capacity (in MW)								900			22,700
Notional Amount of Foreign Currency Derivatives			141
Long Lived Assets Held-for-sale, Proceeds from Sale		174
Gain (Loss) on Derivative Instruments, Net, Pretax	4
Assets Held-for-sale, Long Lived		$ 38