Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenues		$ 6,910		$ 6,514		$ 14,485		$ 15,345
Unrealized Gain (Loss) on Derivatives						(194)		507
Nuclear Fuel Amortization	[1]					557		552
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		2,012		2,383		4,309		4,792
Revenues		3,589		4,232		8,329		10,074
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		828		892		1,669		1,679
Revenues		1,416		1,346		2,936		2,859
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		728		745		1,449		1,448
Revenues		1,083		1,206		2,179		2,482
Generation New England [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		110		88		191		246
Revenues		351		367		821		1,226
Generation New York [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		267		145		395		334
Revenues		346		218		549		528
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		77		70		138		125
Revenues		207		192		370		372
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		96		62		174		108
Revenues		223		289		447		503
Other Segments [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel				381
Other Segments [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel				6				10
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Amortization Of Intangible Assets Related To Commodity Contracts For Revenue Net Purchased Power And Fuel		(12)		(14)		7		24
Segment Reporting Information Revenue Net of Purchase Power And Fuel		(94)				293		852
Revenues		(37)		614		1,027		2,104
Derivative, Gain on Derivative				25				179
Unrealized Gain (Loss) on Derivatives		(304)		235		(201)		397
Nuclear Fuel Amortization		(9)				(9)
Generation All Other Segments [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Intersegment RNF				(6)				(25)
Segment Reporting Information Revenue Net of Purchase Power And Fuel								15
Revenues				46				(89)
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		2,106		2,002		4,016		3,940
Revenues		3,626		3,618		7,302		7,970
Generation Reportable Segments Total [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Intersegment RNF				6				25
Segment Reporting Information Revenue Net of Purchase Power And Fuel								(10)
Revenues				46				89
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		0		0		0		0
Revenues		0		0		0		0
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(2)		1		8		(11)
Revenues		(16)		(18)		(28)	[2]	(61)	[2]
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		4		(5)		6		(6)
Revenues		7		0		13		0
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(8)		(7)		(13)		(31)
Revenues		(1)		0		(2)		(6)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(3)		7		(13)		28
Revenues		(10)		(4)		(24)	[2]	(1)	[2]
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(34)		(21)		(54)		(54)
Revenues		0		(2)		0		(3)
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(27)		(51)		(37)		(77)
Revenues		(9)	[3]	(21)	[3]	(9)	[4]	(19)	[4]
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		70		76		103		151
Revenues		29	[5]	45	[5]	50	[6]	90	[6]
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(70)		(76)		(103)		(151)
Revenues		(29)		(45)		(50)		(90)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		2,012		2,383		4,309		4,792
Revenues		3,589	[7]	4,232	[7]	8,329	[8]	10,074	[8]
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		830		891		1,661		1,690
Revenues		1,432	[7]	1,364	[7]	2,964	[2],[8]	2,920	[2],[8]
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		724		750		1,443		1,454
Revenues		1,076	[7]	1,206	[7]	2,166	[8]	2,482	[8]
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		118		95		204		277
Revenues		352	[7]	367	[7]	823	[8]	1,232	[8]
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		270		138		408		306
Revenues		356	[7]	222	[7]	573	[2],[8]	529	[2],[8]
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		111		91		192		179
Revenues		207	[7]	194	[7]	370	[8]	375	[8]
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		123		113		211		185
Revenues		232	[3],[7]	310	[3],[7]	456	[4],[8]	522	[4],[8]
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		(164)		305		190		701
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net Of Purchase Power And Fuel From External Customers		2,176		2,078		4,119		4,091
Revenues		$ 3,655	[7]	$ 3,663	[7]	$ 7,352	[8]	$ 8,060	[8]

[1] Included in Purchased power and fuel expense on the Registrants’ Consolidated Statements of Operations and Comprehensive Income

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[3] Three Months Ended June 30, 2016 Three Months Ended June 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,432 $(16) $1,416 $1,364 $(18) $1,346Midwest1,076 7 1,083 1,206 — 1,206New England352 (1) 351 367 — 367New York356 (10) 346 222 (4) 218ERCOT207 — 207 194 (2) 192Other Power Regions232 (9) 223 310 (21) 289Total Revenues for Reportable Segments3,655 (29) 3,626 3,663 (45) 3,618Other(b)(66) 29 (37) 569 45 614Total Generation Consolidated Operating Revenues$3,589 $— $3,589 $4,232 $— $4,232

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[5] Three Months Ended June 30, 2016 Three Months Ended June 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,432 $(16) $1,416 $1,364 $(18) $1,346Midwest1,076 7 1,083 1,206 — 1,206New England352 (1) 351 367 — 367New York356 (10) 346 222 (4) 218ERCOT207 — 207 194 (2) 192Other Power Regions232 (9) 223 310 (21) 289Total Revenues for Reportable Segments3,655 (29) 3,626 3,663 (45) 3,618Other(b)(66) 29 (37) 569 45 614Total Generation Consolidated Operating Revenues$3,589 $— $3,589 $4,232 $— $4,232

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[7] Three Months Ended June 30, 2016 Three Months Ended June 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,432 $(16) $1,416 $1,364 $(18) $1,346Midwest1,076 7 1,083 1,206 — 1,206New England352 (1) 351 367 — 367New York356 (10) 346 222 (4) 218ERCOT207 — 207 194 (2) 192Other Power Regions232 (9) 223 310 (21) 289Total Revenues for Reportable Segments3,655 (29) 3,626 3,663 (45) 3,618Other(b)(66) 29 (37) 569 45 614Total Generation Consolidated Operating Revenues$3,589 $— $3,589 $4,232 $— $4,232

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