Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenues		$ 9,002		$ 7,401		$ 23,486		$ 22,746
Unrealized Gain (Loss) on Derivatives						(100)		363
Nuclear Fuel Amortization	[1]					862		841
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		2,446		2,249		6,754		7,041
Revenues		5,035		4,768		13,363		14,841
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		887		997		2,556		2,677
Revenues		1,800		1,632		4,736		4,491
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		781		756		2,229		2,205
Revenues		1,164		1,151		3,343		3,633
Generation New England [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		160		133		350		379
Revenues		451		520		1,272		1,746
Generation New York [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		194		170		592		502
Revenues		323		250		873		778
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		93		111		231		235
Revenues		295		316		665		687
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		77		83		253		193
Revenues		238		376		686		880
Other Segments [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel				(1)
Other Segments [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel				13				(6)
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		(22)		(4)		(15)		20
Segment Reporting Information Revenue Net of Purchase Power And Fuel		254				543		850
Revenues		764		523		1,788		2,626
Derivative, Gain on Derivative		(187)						171
Unrealized Gain (Loss) on Derivatives		88		139		(113)		258
Nuclear Fuel Amortization		(28)				(38)
Generation All Other Segments [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Intersegment RNF				(12)				(22)
Segment Reporting Information Revenue Net of Purchase Power And Fuel								28
Revenues				54				(144)
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Segment Reporting Information Revenue Net of Purchase Power And Fuel		2,192		2,250		6,211		6,191
Revenues		4,271		4,245		11,575		12,215
Generation Reportable Segments Total [Member] | Scenario, Adjustment [Member]
Segment Reporting Information [Line Items]
Intersegment RNF				12				22
Segment Reporting Information Revenue Net of Purchase Power And Fuel								6
Revenues				54				144
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		6		18		15		(2)
Revenues		(13)		(8)		(40)	[2]	(69)	[2]
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(1)		(4)		4		(15)
Revenues		1		(1)		13		(1)
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(10)		(15)		(23)		(46)
Revenues		(4)		0		(6)		(6)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(1)		13		(15)		40
Revenues		(8)		(4)		(33)	[2]	(5)	[2]
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(51)		(55)		(104)		(109)
Revenues		6		(1)		6		(4)
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(66)		(71)		(104)		(148)
Revenues		(33)	[3]	(40)	[3]	(42)	[4]	(60)	[4]
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		123		114		227		280
Revenues		51	[5]	54	[5]	102	[6]	145	[6]
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Intersegment RNF		(123)		(114)		(227)		(280)
Revenues		(51)		(54)		(102)		(145)
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		2,446		2,249		6,754		7,041
Revenues		5,035	[7]	4,768	[7]	13,363	[8]	14,841	[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		881		979		2,541		2,679
Revenues		1,813	[7]	1,640	[7]	4,776	[2],[8]	4,560	[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		782		760		2,225		2,220
Revenues		1,163	[7]	1,152	[7]	3,330	[8]	3,634	[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		170		148		373		425
Revenues		455	[7]	520	[7]	1,278	[8]	1,752	[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		195		157		607		462
Revenues		331	[7]	254	[7]	906	[2],[8]	783	[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		144		166		335		344
Revenues		289	[7]	317	[7]	659	[8]	691	[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		143		154		357		341
Revenues		271	[3],[7]	416	[3],[7]	728	[4],[8]	940	[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		131		(115)		316		570
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		2,315		2,364		6,438		6,471
Revenues		$ 4,322	[7]	$ 4,299	[7]	$ 11,677	[8]	$ 12,360	[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 September 30, 2016 Three Months Ended September 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,813 $(13) $1,800 $1,640 $(8) $1,632Midwest1,163 1 1,164 1,152 (1) 1,151New England455 (4) 451 520 — 520New York331 (8) 323 254 (4) 250ERCOT289 6 295 317 (1) 316Other Power Regions271 (33) 238 416 (40) 376Total Revenues for Reportable Segments4,322 (51) 4,271 4,299 (54) 4,245Other(b)713 51 764 469 54 523Total Generation Consolidated Operating Revenues$5,035 $— $5,035 $4,768 $— $4,768

[4] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjk5ZDczZmJiZmVlMzQwYjliY2U1YWJiOTI2MjM3MWNhfFRleHRTZWxlY3Rpb246NUY3NkI2RTJCMDAyNTZBQUJFNUZBOTU5MTExODhGMDgM}

[5] Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,813 $(13) $1,800 $1,640 $(8) $1,632Midwest1,163 1 1,164 1,152 (1) 1,151New England455 (4) 451 520 — 520New York331 (8) 323 254 (4) 250ERCOT289 6 295 317 (1) 316Other Power Regions271 (33) 238 416 (40) 376Total Revenues for Reportable Segments4,322 (51) 4,271 4,299 (54) 4,245Other(b)713 51 764 469 54 523Total Generation Consolidated Operating Revenues$5,035 $— $5,035 $4,768 $— $4,768

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[7] Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Revenuesfrom externalcustomers(a)IntersegmentrevenuesTotalRevenuesRevenuesfrom externalcustomers(a)(c)Intersegmentrevenues(c)TotalRevenues(c)Mid-Atlantic$1,813 $(13) $1,800 $1,640 $(8) $1,632Midwest1,163 1 1,164 1,152 (1) 1,151New England455 (4) 451 520 — 520New York331 (8) 323 254 (4) 250ERCOT289 6 295 317 (1) 316Other Power Regions271 (33) 238 416 (40) 376Total Revenues for Reportable Segments4,322 (51) 4,271 4,299 (54) 4,245Other(b)713 51 764 469 54 523Total Generation Consolidated Operating Revenues$5,035 $— $5,035 $4,768 $— $4,768

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