Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 2,504	$ 2,431	$ 6,066	$ 6,292
Mark-to-market for economic hedging activities	(64)	(68)	(23)	62
Operating revenues	2,996	2,960	7,626	7,486
Lease revenue	5	6	15	17
Derivative revenue	(68)	(64)	(8)	79
Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	442	445	997	1,298
Derivative revenue	668	418	1,398	1,081
Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	152	192	461	500
Derivative revenue	33	45	81	110
Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,080	1,766	4,800	4,317
Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	465	434	1,096	1,181
Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,545	2,200	5,896	5,498
Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(210)	55	51	(31)
Derivative revenue	487	523	1,545	1,177
Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	67	68	221	221
Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	2,996	2,960	7,626	7,486
Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	(4)	5	15	17
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	(5)	(4)	(26)
Operating revenues	(522)	(722)	(1,437)	(927)
Lease revenue	0	0	0	0
Corporate/Eliminations | Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(568)	(479)	(1,209)	(890)
Derivative revenue	(569)	(475)	(1,211)	(880)
Corporate/Eliminations | Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	1	1	1
Derivative revenue	0	0	1	0
Corporate/Eliminations | Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	(2)	(2)	(4)
Corporate/Eliminations | Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Corporate/Eliminations | Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	(2)	(2)	(4)
Corporate/Eliminations | Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	47	(241)	(223)	(21)
Derivative revenue	(522)	(716)	(1,433)	(901)
Corporate/Eliminations | Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(2)	0	(4)	(13)
Corporate/Eliminations | Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	(522)	(721)	(1,437)	(927)
Corporate/Eliminations | Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	0	0	0	0
Retail | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,542	2,199	5,889	5,492
Operating revenues	2,544	2,203	5,899	5,497
Lease revenue	3	3	9	10
Retail | Operating Segments | Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Derivative revenue	0	0	0	0
Retail | Operating Segments | Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Derivative revenue	0	0	0	0
Retail | Operating Segments | Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,080	1,768	4,802	4,321
Retail | Operating Segments | Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	465	434	1,096	1,181
Retail | Operating Segments | Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,545	2,202	5,898	5,502
Retail | Operating Segments | Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(1)	1	1	(5)
Derivative revenue	(1)	1	1	(5)
Retail | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Retail | Operating Segments | Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	2,544	2,203	5,899	5,497
Retail | Operating Segments | Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	0	0	0	0
Retail | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	(1)	(1)	(6)	(7)
Generation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(38)	237	181	826
Operating revenues	974	1,478	3,164	2,916
Lease revenue	2	3	6	7
Generation | Operating Segments | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(379)	(306)	(736)	(562)
Lease revenue	0	0	0	0
Generation | Operating Segments | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	341	543	917	1,388
Lease revenue	2	3	6	7
Generation | Operating Segments | Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,010	924	2,206	2,188
Derivative revenue	1,237	893	2,609	1,961
Generation | Operating Segments | Energy revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	773	585	1,628	1,251
Derivative revenue	1,175	901	2,417	1,883
Generation | Operating Segments | Energy revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	237	339	578	937
Derivative revenue	62	(8)	192	78
Generation | Operating Segments | Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	151	191	460	499
Derivative revenue	33	45	80	110
Generation | Operating Segments | Capacity revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	1	0	1
Derivative revenue	0	0	0	0
Generation | Operating Segments | Capacity revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	151	190	460	498
Derivative revenue	33	45	80	110
Generation | Operating Segments | Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Retail revenue | Texas | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Retail revenue | Texas | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Retail revenue | East/West/Other | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Retail revenue | East/West/Other | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Total retail revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Total retail revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Generation | Operating Segments | Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(256)	295	273	(5)
Derivative revenue	1,010	1,238	2,977	2,083
Generation | Operating Segments | Derivative revenue | Texas
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(240)	259	233	(14)
Derivative revenue	944	1,159	2,674	1,872
Generation | Operating Segments | Derivative revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(16)	36	40	9
Derivative revenue	66	79	303	211
Generation | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	69	68	225	234
Generation | Operating Segments | Other revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	32	8	77	72
Generation | Operating Segments | Other revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	37	60	148	162
Generation | Operating Segments | Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	974	1,478	3,164	2,916
Generation | Operating Segments | Total operating revenue | Texas
Disaggregation of Revenue [Line Items]
Operating revenues	565	853	1,938	1,310
Generation | Operating Segments | Total operating revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Operating revenues	409	625	1,226	1,606
Generation | Operating Segments | Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	(4)	5	15	17
Generation | Operating Segments | Other revenue, derivative | Texas
Disaggregation of Revenue [Line Items]
Derivative revenue	9	(1)	24	3
Generation | Operating Segments | Other revenue, derivative | East/West/Other
Disaggregation of Revenue [Line Items]
Derivative revenue	(13)	6	(9)	14
Generation | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	$ (513)	$ (739)	$ (1,375)	$ (944)