Accounting for Derivative Instruments and Hedging Activities (Details 3) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income
Accumulated OCI beginning balance			$ (101)	$ (68)	$ (23)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			21	15	13
Mark-to-market of cash flow hedge accounting contracts			14	(48)	(58)
Accumulated OCI ending balance, net of tax			(66)	(101)	(68)
Accumulated OCI ending balance, tax			16	16	35
Losses expected to be realized from OCI during the next 12 months, net of $3 tax			(16)
Gains/(losses) expected to be realized from OCI during the next 12 months, tax			4
Unrealized mark-to-market results
Reversal of previously recognized unrealized gains on settled positions related to economic hedges			(245)	(275)	(15)
Reversal of acquired gain positions related to economic hedges			(60)	(106)	(333)
Net unrealized gains on open positions related to economic hedges			20	9	361
Losses on ineffectiveness associated with open positions treated as cash flow hedges			0	0	0
Total unrealized mark-to-market (losses)/gains for economic hedging activities			(285)	(372)	13
Reversal of previously recognized unrealized losses/(gains) on settled positions related to trading activity			10	(46)	1
Reversal of Previously Unrecognized Unrealized Gain Loss Acquired as Part of Acquisition Trading Activity			0	(14)	(32)
Net unrealized gains/(losses) on open positions related to trading activity			18	(16)	45
Total unrealized mark-to-market gains/(losses) for trading activity			28	(76)	14
Total unrealized (losses)/gains			(257)	(448)	27
Impact of derivative instruments to statement of operations
Total unrealized (losses)/gains			(257)	(448)	27
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions			36
Collateral required for contracts with credit rating contingent features			56
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty			14
Realized Gain [Member]
Unrealized mark-to-market results
Derivative Instruments Not Designated as Hedging Instruments, Gain	$ 98	$ 38
Energy commodities
Accumulated Other Comprehensive Income
Accumulated OCI beginning balance			0	(1)	(1)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net				1	0
Mark-to-market of cash flow hedge accounting contracts				0	0
Accumulated OCI ending balance, net of tax				0	(1)
Unrealized mark-to-market results
Total unrealized (losses)/gains			(257)	(448)	27
Impact of derivative instruments to statement of operations
Total unrealized (losses)/gains			(257)	(448)	27
Energy commodities | Unrealized (losses)/gains included in operating revenues
Unrealized mark-to-market results
Total unrealized (losses)/gains			(837)	(320)	515
Impact of derivative instruments to statement of operations
Total unrealized (losses)/gains			(837)	(320)	515
Energy commodities | Unrealized gains/(losses) included in cost of operations
Unrealized mark-to-market results
Total unrealized (losses)/gains			580	(128)	(488)
Impact of derivative instruments to statement of operations
Total unrealized (losses)/gains			580	(128)	(488)
Interest rate contracts
Accumulated Other Comprehensive Income
Accumulated OCI beginning balance			(101)	(67)	(22)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			21	14	13
Mark-to-market of cash flow hedge accounting contracts			14	(48)	(58)
Accumulated OCI ending balance, net of tax			(66)	(101)	(67)
Losses expected to be realized from OCI during the next 12 months, net of $3 tax			(16)
Unrealized mark-to-market results
Total unrealized (losses)/gains			36	17	(31)
Impact of derivative instruments to statement of operations
Total unrealized (losses)/gains			$ 36	17	$ (31)
NRG Solar Dandan [Member] | Interest rate contracts
Discontinuation of Cash Flow Hedge [Abstract]
Loss previously deferred in OCI recognized in earnings resulting from discontinued cash flow hedge accounting				$ 6
2016 [Member] | Realized Gain [Member]
Unrealized mark-to-market results
Derivative Instruments Not Designated as Hedging Instruments, Gain		18
2017 [Member] | Realized Gain [Member]
Unrealized mark-to-market results
Derivative Instruments Not Designated as Hedging Instruments, Gain	82	19
2018 [Member] | Realized Gain [Member]
Unrealized mark-to-market results
Derivative Instruments Not Designated as Hedging Instruments, Gain	13	$ 1
2019 [Member] | Realized Gain [Member]
Unrealized mark-to-market results
Derivative Instruments Not Designated as Hedging Instruments, Gain	$ 3