Fair Value of Financial Instruments (Details 4) (USD $)	3 Months Ended
	Jun. 30, 2013		Jun. 30, 2012
Concentration of Credit Risk
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent)	7.00%
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent)	10.00%
Increase in fair value as a result of the credit reserve	$ 1,000,000		$ (9,000,000)
Increase in fair value as a result of the credit reserve, portion recorded in OCI	(2,000,000)		(6,000,000)
Increase in fair value as a result of the credit reserve, portion recorded in operating revenue and cost of operations	3,000,000		(3,000,000)
Counterparty credit exposure to a portion of the Company's counterparties	1,300,000,000
Collateral held (cash and letters of credit) against counterparty credit exposure to a portion of the Company's counterparties	76,000,000
Net counterparty credit exposure to a portion of the Company's counterparties	1,200,000,000
Company's exposure before collateral is expected to roll off by the end of 2014 (as a percent)	87.00%
Net Exposure (as a percent)	100.00%	[1]
Counterparty credit risk exposure to certain counterparties, threshold (as a percent)	10.00%
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	515,000,000
Estimated counterparty credit risk exposure under certain long term agreements, including California tolling agreements, South Central load obligations and solar power purchase agreements for the next 5 years	$ 1,700,000,000
Period of estimated counterparty credit risk exposure under certain long term agreements, including California tolling agreements, South Central load obligations and solar power purchase agreements (in years)	5 years
Investment grade
Concentration of Credit Risk
Net Exposure (as a percent)	92.00%	[1]
Non-rated
Concentration of Credit Risk
Net Exposure (as a percent)	7.00%	[1],[2]
Non-investment grade
Concentration of Credit Risk
Net Exposure (as a percent)	1.00%	[1]
Financial institutions
Concentration of Credit Risk
Net Exposure (as a percent)	48.00%	[1]
Utilities, energy merchants, marketers and other
Concentration of Credit Risk
Net Exposure (as a percent)	35.00%	[1]
ISOs
Concentration of Credit Risk
Net Exposure (as a percent)	16.00%	[1]
Coal and emissions
Concentration of Credit Risk
Net Exposure (as a percent)	1.00%	[1]

[1] Counterparty credit exposure excludes uranium and coal transportation contracts because of the unavailability of market prices.

[2] For non-rated counterparties, a significant portion are related to ISO and municipal public power entities, which are considered investment grade equivalent ratings based on NRG's internal credit ratings.