Segment Information - Generation Total Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 8,343	$ 8,929	$ 7,689	$ 9,477	$ 8,812	$ 9,401	$ 8,074	$ 9,691	$ 34,438	$ 35,978	$ 33,558
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues									5,074	5,487	5,540
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues									4,293	4,706	4,181
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues									1,596	1,687	1,550
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues									1,013	1,133	962
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues									4,246	4,335	3,051
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues									16,222	17,348	15,284
Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues									2,210	2,780	2,628
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues									492	309	588
Unrealized Gain (Loss) on Securities									(4)	(262)	(131)
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues									18,924	20,437	18,500
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues									34,438	35,978	33,560
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues									5,070	5,474	5,515
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues									4,327	4,717	4,206
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues									1,596	1,687	1,567
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues									997	1,132	958
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues									4,295	4,401	3,086
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues									16,285	17,411	15,332
Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues									2,148	2,718	2,575
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Amortization of intangible assets related to commodity contracts											38
Unrealized Gain (Loss) on Securities									(215)	(319)	(175)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues									18,924	20,437	18,500
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues									(3,166)	(3,294)	(3,025)
Revenue from Related Parties									2	1	2
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									4	13	25
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									(34)	(11)	(25)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									0	0	(17)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									16	1	4
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									(49)	(66)	(35)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									(63)	(63)	(48)
Intersegment Eliminations [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									62	62	53
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue from Related Parties									1	1	(5)
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									491	308	593
Contracts with Customers [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues									5,053	5,241	5,523
Contracts with Customers [Member] | Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues									4,095	4,527	3,923
Contracts with Customers [Member] | Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues									1,571	1,723	1,605
Contracts with Customers [Member] | Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues									768	572	641
Contracts with Customers [Member] | Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues									3,687	3,530	2,658
Contracts with Customers [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues									15,174	15,593	14,350
Contracts with Customers [Member] | Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues									1,446	1,524	1,658
Contracts with Customers [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues									17,060	17,627	16,752
Contracts with Customers [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues									440	510	744
Other [Member] | Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenues									17	233	(8)
Other [Member] | Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenues									232	190	283
Other [Member] | Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenues									25	(36)	(38)
Other [Member] | Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenues									229	560	317
Other [Member] | Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenues									608	871	428
Other [Member] | Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenues									1,111	1,818	982
Other [Member] | Operating Segments [Member] | Generation Natural Gas [Member]
Segment Reporting Information [Line Items]
Revenues									702	1,194	917
Other [Member] | Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenues									1,864	2,810	1,748
Other [Member] | Corporate, Non-Segment [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues									$ 51	$ (202)	$ (151)