Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 8,910	$ 8,853	$ 26,715	$ 24,925
Revenue from Related Parties	0	1	0	4
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,272	1,313	3,527	3,561
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	985	1,043	2,945	3,007
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	455	406	1,173	1,061
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	358	330	890	754
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	1,260	1,109	3,729	2,984
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	4,330	4,201	12,264	11,367
Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	575	323	2,408	1,348
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	(499)	135	(555)	557
Unrealized Gain (Loss) on Securities	(635)	37	(958)	238
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,406	4,659	14,117	13,272
Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,268	1,307	3,511	3,540
Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	985	1,042	2,944	3,013
Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	455	406	1,174	1,062
Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	356	323	879	734
Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	1,266	1,123	3,756	3,018
Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	4,330	4,201	12,264	11,367
Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	575	323	2,408	1,348
Operating Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Securities	754	255	1,242	472
Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,406	4,659	14,117	13,272
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	(499)	135	(555)	557
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(877)	(845)	(2,470)	(2,430)
Revenue from Related Parties	(876)	(845)	(2,469)	(2,430)
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	4	6	16	21
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	1	1	(6)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	(1)	(1)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	2	7	11	20
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(6)	(14)	(27)	(34)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	0	0
Intersegment Eliminations [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	0	0
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	0	0
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	0	0
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,145	1,327	3,377	3,692
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	1,084	974	3,067	2,773
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	445	401	1,204	1,074
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	191	249	724	579
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	948	937	3,043	2,718
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	3,813	3,888	11,415	10,836
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	266	169	1,384	881
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,174	4,142	13,090	11,985
Revenue from Contract with Customer [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	95	85	291	268
Other [Member] | Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	123	(20)	134	(152)
Other [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	(99)	68	(123)	240
Other [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	10	5	(30)	(12)
Other [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	165	74	155	155
Other [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	318	186	713	300
Other [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	517	313	849	531
Other [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	309	154	1,024	467
Other [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	232	517	1,027	1,287
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	$ (594)	$ 50	$ (846)	$ 289