Accounting for Derivative Instruments and Hedging Activities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative
Derivative Liability, Fair Value, Gross Liability	$ 3,023		$ 3,023		$ 4,215
Derivative Asset, Fair Value, Gross Asset	2,986		2,986		4,666
Effects of ASC 815 on NRG's Accumulated OCI balance attributable to cash flow hedge derivatives, net of tax
Accumulated OCI beginning balance	43	292	132	441
Due to realization of previously deferred amounts	(27)	(91)	(95)	(270)
Mark-to-market of cash flow hedge accounting contracts	(16)	15	(37)	45
Accumulated OCI ending balance, net of tax	0	216	0	216
Gains/(losses) expected to be realized from OCI during the next 12 months, net of tax	66	184	66	184
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges	85	(50)	160	(72)
Reversal of loss positions acquired as part of the acquisition	15	11	(5)	(60)
Net unrealized gains/(losses) on open positions related to economic hedges	(159)	(7)	(78)	77
Losses on ineffectiveness associated with open positions treated as cash flow hedges	0	9	(51)	8
Total unrealized mark-to-market gains/(losses) for economic hedging activities	(259)	41	(284)	217
Reversal of previously recognized unrealized (gains) losses on settled positions related to trading activity	15	(8)	45	(22)
Net unrealized gains on open positions related to trading activity	(3)	0	33	22
Total unrealized mark-to-market gains for trading activity	(18)	8	(12)	44
Total unrealized gains/(losses)	(277)	49	(296)	261
Impact of derivative instruments to statement of operations
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	9	(51)	11
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions	91		91
Collateral required for contracts with credit rating contingent features in net liability position	51		51
Marginable agreements counterparty has not called for the collateral in net liability position	56		56
Cost of operations
Unrealized mark-to-market results
Total unrealized gains/(losses)	118	(40)	174	68
Commodity contracts
Unrealized mark-to-market results
Total unrealized gains/(losses)	(277)	49	(296)	261
Interest rate contracts
Unrealized mark-to-market results
Total unrealized gains/(losses)	0	(1)	(12)	2
Alpine Financing Agreement
Effects of ASC 815 on NRG's Accumulated OCI balance attributable to cash flow hedge derivatives, net of tax
Due to realization of previously deferred amounts			4
Commodity contracts
Effects of ASC 815 on NRG's Accumulated OCI balance attributable to cash flow hedge derivatives, net of tax
Accumulated OCI beginning balance	111	332	188	488
Due to realization of previously deferred amounts	(30)	(91)	(106)	(281)
Mark-to-market of cash flow hedge accounting contracts	(1)	19	(2)	53
Accumulated OCI ending balance, net of tax	80	260	80	260
Gains/(losses) expected to be realized from OCI during the next 12 months, net of tax	77	186	77	186
Impact of derivative instruments to statement of operations
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	9	(51)	8
Commodity contracts | Revenue (expense) from operations
Unrealized mark-to-market results
Total unrealized gains/(losses)	(395)	89	(470)	193
Interest Rate
Effects of ASC 815 on NRG's Accumulated OCI balance attributable to cash flow hedge derivatives, net of tax
Accumulated OCI beginning balance	(68)	(40)	(56)	(47)
Due to realization of previously deferred amounts	3	0	11	11
Mark-to-market of cash flow hedge accounting contracts	(15)	(4)	(35)	(8)
Accumulated OCI ending balance, net of tax	(80)	(44)	(80)	(44)
Gains/(losses) expected to be realized from OCI during the next 12 months, net of tax	(11)	(2)	(11)	(2)
Impact of derivative instruments to statement of operations
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	0	0	0	3
Designated as Hedging Instrument [Member]
Derivative
Derivative Liability, Fair Value, Gross Liability	110		110		108
Derivative Asset, Fair Value, Gross Asset	1		1		318
Designated as Hedging Instrument [Member] | Interest rate contracts current
Derivative
Derivative Liability, Fair Value, Gross Liability	11		11		39
Derivative Asset, Fair Value, Gross Asset	0		0		0
Designated as Hedging Instrument [Member] | Interest rate contracts long-term
Derivative
Derivative Liability, Fair Value, Gross Liability	96		96		68
Derivative Asset, Fair Value, Gross Asset	0		0		0
Designated as Hedging Instrument [Member] | Commodity contracts current
Derivative
Derivative Liability, Fair Value, Gross Liability	2		2		0
Derivative Asset, Fair Value, Gross Asset	1		1		318
Designated as Hedging Instrument [Member] | Commodity contracts long-term
Derivative
Derivative Liability, Fair Value, Gross Liability	1		1		1
Derivative Asset, Fair Value, Gross Asset	0		0		0
Not Designated as Hedging Instrument [Member]
Derivative
Derivative Liability, Fair Value, Gross Liability	2,913		2,913		4,107
Derivative Asset, Fair Value, Gross Asset	2,985		2,985		4,348
Not Designated as Hedging Instrument [Member] | Interest rate contracts current
Derivative
Derivative Liability, Fair Value, Gross Liability	13		13		0
Derivative Asset, Fair Value, Gross Asset	0		0		0
Not Designated as Hedging Instrument [Member] | Interest rate contracts long-term
Derivative
Derivative Liability, Fair Value, Gross Liability	14		14		1
Derivative Asset, Fair Value, Gross Asset	0		0		0
Not Designated as Hedging Instrument [Member] | Commodity contracts current
Derivative
Derivative Liability, Fair Value, Gross Liability	2,436		2,436		3,712
Derivative Asset, Fair Value, Gross Asset	2,676		2,676		3,898
Not Designated as Hedging Instrument [Member] | Commodity contracts long-term
Derivative
Derivative Liability, Fair Value, Gross Liability	450		450		394
Derivative Asset, Fair Value, Gross Asset	$ 309		$ 309		$ 450