Environmental Matters (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012
Environmental Matters Disclosure [Abstract]
Estimated environmental capital expenditures from 2011 through 2015	$ 440	$ 553

Amount of estimated environmental capital expenditures to be incurred during the remainder of the current fiscal year and for the next four years to meet the company's environmental commitments.

No definition available.

Name:	nrg_EnvironmentalCapitalExpendituresEstimated
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant