Accounting for Derivative Instruments and Hedging Activities - Effects on Accumulated OCL balance attributable to hedge derivatives, net (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Derivative [Line Items]
Beginning balance	$ 2,071	$ 1,968
Reclassified from accumulated OCL to income:
Sale of NRG Yield and Renewables Platform	25	25
Ending balance	(936)	(936)
Balance, tax	0	0
AOCI Including Portion Attributable to Noncontrolling Interest | Interest rate contracts
Derivative [Line Items]
Beginning balance	(23)	(54)
Reclassified from accumulated OCL to income:
Ending balance	0	0
Accumulated Gain (Loss), Net, Cash Flow Hedge | Interest rate contracts
Reclassified from accumulated OCL to income:
Due to realization of previously deferred amounts	1	8
Accumulated Net Gain (Loss) from Cash Flow Hedges | Interest rate contracts
Reclassified from accumulated OCL to income:
Mark-to-market of cash flow hedge accounting contracts	$ (3)	$ 21