Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 8,853	$ 8,929	$ 24,925	$ 26,096
Revenue from Related Parties	1	0	4	0
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,313	1,364	3,561	3,809
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	1,043	1,082	3,007	3,224
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	406	429	1,061	1,211
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	330	365	754	805
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	1,109	1,040	2,984	3,254
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	4,201	4,280	11,367	12,303
Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	323	299	1,348	1,541
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	135	195	557	436
Unrealized Gain (Loss) on Securities	37	77	238	64
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,659	4,774	13,272	14,280
Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	1,307	1,361	3,540	3,807
Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	1,042	1,099	3,013	3,255
Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	406	429	1,062	1,211
Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	323	360	734	792
Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	1,123	1,065	3,018	3,300
Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	4,201	4,314	11,367	12,365
Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	323	265	1,348	1,479
Operating Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Unrealized Gain (Loss) on Securities	255	17	472	(84)
Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,659	4,774	13,272	14,280
Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	135	195	557	436
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(845)	(763)	(2,430)	(2,296)
Revenue from Related Parties	(845)	(764)	(2,430)	(2,300)
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	6	3	21	2
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	1	(17)	(6)	(31)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	0	(1)	0
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	7	5	20	13
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	(14)	(25)	(34)	(46)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	(34)	0	(62)
Intersegment Eliminations [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties	0	34	0	62
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	1,327	1,351	3,692	3,798
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	974	1,052	2,773	3,083
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	401	414	1,074	1,195
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	249	288	579	594
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	937	873	2,718	2,849
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	3,888	3,978	10,836	11,519
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	169	160	881	1,041
Revenue from Contract with Customer [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	4,142	4,250	11,985	12,903
Revenue from Contract with Customer [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	85	112	268	343
Other [Member] | Operating Segment [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues	(20)	10	(152)	9
Other [Member] | Operating Segment [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues	68	47	240	172
Other [Member] | Operating Segment [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues	5	15	(12)	16
Other [Member] | Operating Segment [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues	74	72	155	198
Other [Member] | Operating Segment [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues	186	192	300	451
Other [Member] | Operating Segment [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues	313	336	531	846
Other [Member] | Operating Segment [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues	154	105	467	438
Other [Member] | Operating Segment [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues	517	524	1,287	1,377
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues	$ 50	$ 83	$ 289	$ 93