Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	$ 735		$ 783		$ 1,586		$ 1,557
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	772		728		1,631		1,443
Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	96		147		216		257
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	266		270		549		415
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	82		70		118		138
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	90		90		208		152
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,041		2,088		4,308		3,962
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	258	[1]	(29)	[1]	209	[2]	176	[2]
Unrealized Gain (Loss) on Securities	(5)		(143)		(102)		(98)
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,299		2,059		4,517		4,138
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	722	[3]	757	[3]	1,558	[4]	1,513	[4]
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	770	[3]	728	[3]	1,617	[4]	1,431	[4]
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	104	[3]	157	[3]	227	[4]	271	[4]
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	259	[3]	270	[3]	541	[4]	415	[4]
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	129	[3]	121	[3]	235	[4]	214	[4]
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	125	[3]	134	[3]	284	[4]	240	[4]
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,109	[3]	2,167	[3]	4,462	[4]	4,084	[4]
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	190	[1],[3]	(108)	[1],[3]	55	[2],[4]	54	[2],[4]
Amortization Of Intangible Assets Related To Commodity Contracts For Revenue Net Purchased Power And Fuel			(20)				(22)
Unrealized Gain (Loss) on Securities	90		(184)		(175)		(233)
Nuclear Fuel Amortization	(20)		(2)		(34)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,299	[3]	2,059	[3]	4,517	[4]	4,138	[4]
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	13		26		28		44
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	2		0		14		12
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(8)		(10)		(11)		(14)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	7		0		8		0
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(47)		(51)		(117)		(76)
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(35)		(44)		(76)		(88)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(68)		(79)		(154)		(122)
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	68	[1]	79	[1]	154	[2]	122	[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 decrease to RNF for the amortization of intangible assets and liabilities related to commodity contracts for the three months ended June 30, 2017, unrealized mark-to-market gains of $90 million and losses of $184 million for the three months ended June 30, 2018 and 2017, respectively, accelerated nuclear fuel amortization associated with announced early plant retirements as discussed in Note 8 - Early Plant Retirements of $20 million decrease and $2 million decrease to revenue net of purchased power and fuel expense for the three months ended June 30, 2018, and 2017, 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 for the amortization of intangible assets and liabilities related to commodity contracts for the six months ended June 30, 2017, unrealized mark-to-market losses of $175 million and $233 million for the six months ended June 30, 2018 and 2017, respectively, accelerated nuclear fuel amortization associated with announced early plant retirements as discussed in Note 8 - Early Plant Retirements of $34 million decrease to revenue net of purchased power and fuel expense for the six months ended June 30, 2018, 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.