Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	$ 783		$ 828		$ 1,557		$ 1,669
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	728		728		1,443		1,449
Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	147		110		257		191
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	230		267		385		395
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	70		77		138		138
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	90		96		152		174
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,048		2,106		3,932		4,016
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	(31)	[1]	(94)	[1]	174	[2]	293	[2]
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,017		2,012		4,106		4,309
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	757	[3]	830	[3]	1,513	[4]	1,661	[4]
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	728	[3]	724	[3]	1,431	[4]	1,443	[4]
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	157	[3]	118	[3]	271	[4]	204	[4]
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	230	[3]	270	[3]	385	[4]	408	[4]
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	121	[3]	111	[3]	214	[4]	192	[4]
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	134	[3]	123	[3]	240	[4]	211	[4]
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,127	[3]	2,176	[3]	4,054	[4]	4,119	[4]
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	(110)	[1],[3]	(164)	[1],[3]	52	[2],[4]	190	[2],[4]
Amortization Of Intangible Assets Related To Commodity Contracts For Revenue Net Purchased Power And Fuel	(20)		(12)		(22)		7
Unrealized Gain on Securities	(184)		(304)		(233)		(201)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,017	[3]	2,012	[3]	4,106	[4]	4,309	[4]
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	26		(2)		44		8
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	0		4		12		6
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(10)		(8)		(14)		(13)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	0		(3)		0		(13)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(51)		(34)		(76)		(54)
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(44)		(27)		(88)		(37)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(79)		(70)		(122)		(103)
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	79	[1]	70	[1]	122	[2]	103	[2]
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	$ 0		$ 0		$ 0		$ 0

[1] Other represents activities not allocated to a region. See text above for a description of included activities. Includes a $20 million and $12 million decrease to RNF for the amortization of intangible assets and liabilities related to commodity contracts for the three months ended June 30, 2017 and 2016, respectively, unrealized mark-to-market losses of $184 million and $304 million for the three months ended June 30, 2017 and 2016, respectively, and the elimination of intersegment revenue net of purchased power and fuel expense.

[2] Other represents activities not allocated to a region. See text above for a description of included activities. Includes a $22 million decrease to RNF and a $7 million increase to RNF for the amortization of intangible assets and liabilities related to commodity contracts for the six months ended June 30, 2017 and 2016, respectively, unrealized mark-to-market losses of $233 million and $201 million for the six months ended June 30, 2017 and 2016, respectively, and the elimination of intersegment revenue net of purchased power and fuel expense.

[3] Includes purchases and sales from/to third parties and affiliated sales to the Utility Registrants.

[4] Includes purchases and sales from/to third parties and affiliated sales to the Utility Registrants.