Variable Interest Entities, or VIEs (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011 states facility generatingunit	Dec. 31, 2010
Investments Accounted for by the Equity Method
Number of peaking facilities to be constructed	2
Equity investments in affiliates	$ 576	$ 536
Number of Generation Facilities Owned	2
Number of Generating Units at Middletown Facilitiy	4
Sherbino I Wind Farm LLC
Investments Accounted for by the Equity Method
Economic interest in equity method investments (as a percent)	50.00%
Maximum exposure to loss, investment in Sherbino	94
NRG Connecticut Peaking LLC | GenConn Energy LLC
Investments Accounted for by the Equity Method
Economic interest in equity method investments (as a percent)	50.00%
GenConn Energy LLC
Investments Accounted for by the Equity Method
Power generation capacity of the peaking facility to be constructed in Connecticut (in MW)	0
Equity investments in affiliates	134
NRG Connecticut Peaking LLC
Investments Accounted for by the Equity Method
Note receivable due from GenConn	$ 63