Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 33,039	$ 34,438	$ 35,978
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	4,645	5,074	5,487
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	4,024	4,293	4,706
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	1,431	1,596	1,687
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	958	1,013	1,133
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	4,002	4,246	4,335
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	15,060	16,222	17,348
Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	2,003	2,210	2,780
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	540	492	309
Unrealized Gain (Loss) on Securities	110	(4)	(262)
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	17,603	18,924	20,437
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues	33,039	34,438	35,978
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	4,617	5,070	5,474
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	4,029	4,327	4,717
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	1,432	1,596	1,687
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	933	997	1,132
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	4,049	4,295	4,401
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	15,060	16,285	17,411
Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	2,003	2,148	2,718
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Securities	295	(215)	(319)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	17,603	18,924	20,437
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(3,322)	(3,166)	(3,294)
Revenue from Related Parties	4	2	1
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	28	4	13
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(5)	(34)	(11)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(1)	0	0
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	25	16	1
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(47)	(49)	(66)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	(63)	(63)
Intersegment Eliminations [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	62	62
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	1	1
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	0
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	540	491	308
Contracts with Customers [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	4,785	5,053	5,241
Contracts with Customers [Member] | Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	3,717	4,095	4,527
Contracts with Customers [Member] | Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	1,444	1,571	1,723
Contracts with Customers [Member] | Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	735	768	572
Contracts with Customers [Member] | Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	3,586	3,687	3,530
Contracts with Customers [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	14,267	15,174	15,593
Contracts with Customers [Member] | Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	1,283	1,446	1,524
Contracts with Customers [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	15,905	17,060	17,627
Contracts with Customers [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	355	440	510
Other [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	(168)	17	233
Other [Member] | Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	312	232	190
Other [Member] | Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	(12)	25	(36)
Other [Member] | Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	198	229	560
Other [Member] | Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	463	608	871
Other [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	793	1,111	1,818
Other [Member] | Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	720	702	1,194
Other [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	1,698	1,864	2,810
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	$ 185	$ 51	$ (202)