Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,783	$ 2,054	$ 3,562	$ 3,859
Mark-to-market for economic hedging activities	21	(76)	41	130
Operating revenues	2,465	2,461	4,630	4,526
Lease revenue	5	5	10	11
Derivative revenue	33	(71)	60	143
Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	249	410	555	853
Derivative revenue	394	348	730	664
Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	155	165	309	307
Derivative revenue	30	39	48	65
Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,400	1,376	2,720	2,551
Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	345	437	631	747
Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,745	1,813	3,351	3,298
Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	241	10	261	(86)
Derivative revenue	677	402	1,058	656
Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	75	63	154	154
Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	2,465	2,461	4,630	4,526
Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	12	5	19	13
Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(1)	(9)	(4)	(22)
Operating revenues	(650)	(520)	(915)	(206)
Lease revenue	0	0	0	0
Corporate/Eliminations | Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(365)	(251)	(641)	(411)
Derivative revenue	(365)	(247)	(641)	(404)
Corporate/Eliminations | Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1	0	0	(1)
Derivative revenue	1	0	0	0
Corporate/Eliminations | Retail revenue | Mass customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(1)	(1)	(2)	(2)
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	(1)	(1)	(2)	(2)
Corporate/Eliminations | Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	(285)	(264)	(270)	220
Derivative revenue	(649)	(511)	(911)	(184)
Corporate/Eliminations | Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	(4)	(2)	(12)
Corporate/Eliminations | Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	(650)	(520)	(915)	(206)
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	1,743	1,811	3,347	3,293
Operating revenues	1,748	1,814	3,355	3,294
Lease revenue	3	3	6	7
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	1,401	1,377	2,722	2,553
Retail | Operating Segments | Retail revenue | Business Solutions customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	345	437	631	747
Retail | Operating Segments | Total retail revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,746	1,814	3,353	3,300
Retail | Operating Segments | Derivative revenue
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	2	0	2	(6)
Derivative revenue	2	0	2	(6)
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	1,748	1,814	3,355	3,294
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	(2)	(4)	(5)	(6)
Generation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41	252	219	588
Operating revenues	1,367	1,167	2,190	1,438
Lease revenue	2	2	4	4
Generation | Operating Segments | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	(211)	(157)	(357)	(257)
Lease revenue	0	0	0	0
Generation | Operating Segments | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	252	409	576	845
Lease revenue	2	2	4	4
Generation | Operating Segments | Energy revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	614	661	1,196	1,264
Derivative revenue	759	595	1,371	1,068
Generation | Operating Segments | Energy revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	497	402	855	666
Derivative revenue	717	569	1,242	982
Generation | Operating Segments | Energy revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	117	259	341	598
Derivative revenue	42	26	129	86
Generation | Operating Segments | Capacity revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	154	165	309	308
Derivative revenue	29	39	48	65
Generation | Operating Segments | Capacity revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Derivative revenue	0	0	0	0
Generation | Operating Segments | Capacity revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	154	165	309	308
Derivative revenue	29	39	48	65
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	524	274	529	(300)
Derivative revenue	1,324	913	1,967	846
Generation | Operating Segments | Derivative revenue | Texas
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	460	296	473	(273)
Derivative revenue	1,184	865	1,730	714
Generation | Operating Segments | Derivative revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Mark-to-market for economic hedging activities	64	(22)	56	(27)
Derivative revenue	140	48	237	132
Generation | Operating Segments | Other revenue
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	75	67	156	166
Generation | Operating Segments | Other revenue | Texas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16	10	45	64
Generation | Operating Segments | Other revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	59	57	111	102
Generation | Operating Segments | Total operating revenue
Disaggregation of Revenue [Line Items]
Operating revenues	1,367	1,167	2,190	1,438
Generation | Operating Segments | Total operating revenue | Texas
Disaggregation of Revenue [Line Items]
Operating revenues	973	708	1,373	457
Generation | Operating Segments | Total operating revenue | East/West/Other
Disaggregation of Revenue [Line Items]
Operating revenues	394	459	817	981
Generation | Operating Segments | Other revenue, derivative
Disaggregation of Revenue [Line Items]
Derivative revenue	12	5	19	13
Generation | Operating Segments | Other revenue, derivative | Texas
Disaggregation of Revenue [Line Items]
Derivative revenue	7	0	15	5
Generation | Operating Segments | Other revenue, derivative | East/West/Other
Disaggregation of Revenue [Line Items]
Derivative revenue	5	5	4	8
Generation | Corporate/Eliminations
Disaggregation of Revenue [Line Items]
Operating revenues	$ (627)	$ (514)	$ (862)	$ (205)