Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 7,322	$ 7,689	$ 16,069	$ 17,166
Revenue from Related Parties	1	0	2	0
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,074	1,189	2,247	2,445
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	962	1,034	1,964	2,142
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	340	390	655	782
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	234	229	424	441
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	804	861	1,875	2,214
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	3,414	3,703	7,165	8,024
Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	353	348	1,025	1,242
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	113	159	423	240
Unrealized Gain (Loss) on Securities	21	38	200	14
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	3,880	4,210	8,613	9,506
Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,065	1,183	2,232	2,446
Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	962	1,042	1,970	2,156
Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	341	390	656	782
Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	227	225	411	433
Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	819	878	1,896	2,235
Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	3,414	3,718	7,165	8,052
Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	353	333	1,025	1,214
Operating Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Securities	85	74	218	102
Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	3,880	4,210	8,613	9,506
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	113	159	423	240
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(760)	(751)	(1,587)	(1,537)
Revenue from Related Parties	(760)	(750)	(1,585)	(1,534)
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	9	6	15	(1)
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	(8)	(6)	(14)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(1)	0	(1)	0
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	7	4	13	8
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(15)	(17)	(21)	(21)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	(15)	0	(28)
Intersegment Eliminations [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	15	0	28
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	1,100	1,162	2,365	2,448
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	855	974	1,798	2,030
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	336	373	672	781
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	175	178	330	307
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	776	814	1,782	1,976
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	3,242	3,501	6,947	7,542
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	209	177	712	763
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	3,537	3,786	7,844	8,535
Revenue from Contract with Customer [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	86	108	185	230
Other [Member] | Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	(35)	21	(133)	(2)
Other [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	107	68	172	126
Other [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	5	17	(16)	1
Other [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	52	47	81	126
Other [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	43	64	114	259
Other [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	172	217	218	510
Other [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	144	156	313	451
Other [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	343	424	769	971
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	$ 27	$ 51	$ 238	$ 10