Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	$ 763		$ 855		$ 2,348		$ 2,411
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	768		697		2,400		2,140
Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	81		145		298		403
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	292		295		841		707
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	98		118		216		258
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	99		68		309		220
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,101		2,178		6,412		6,139
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	197	[1]	241	[1]	404	[2]	418	[2]
Unrealized Gain (Loss) on Securities	6		52		(96)		(47)
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net of Purchase Power And Fuel	2,298		2,419		6,816		6,557
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	746	[3]	817	[3]	2,303	[4]	2,330	[4]
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	763	[3]	697	[3]	2,381	[4]	2,129	[4]
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	83	[3]	151	[3]	310	[4]	423	[4]
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	290	[3]	295	[3]	832	[4]	708	[4]
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	161	[3]	229	[3]	396	[4]	446	[4]
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	143	[3]	118	[3]	430	[4]	359	[4]
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,186	[3]	2,307	[3]	6,652	[4]	6,395	[4]
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	112	[1],[3]	112	[1],[3]	164	[2],[4]	162	[2],[4]
Amortization Of Intangible Assets Related To Commodity Contracts For Revenue Net Purchased Power And Fuel			(19)				(41)
Unrealized Gain (Loss) on Securities	71		73		(104)		(161)
Nuclear Fuel Amortization	(18)		(6)		(53)		(8)
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue Net Of Purchase Power And Fuel From External Customers	2,298	[3]	2,419	[3]	6,816	[4]	6,557	[4]
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	17		38		45		81
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	5		0		19		11
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(2)		(6)		(12)		(20)
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	2		0		9		(1)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(63)		(111)		(180)		(188)
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(44)		(50)		(121)		(139)
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	(85)		(129)		(240)		(256)
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Intersegment Revenue Net Of Purchase Power And Fuel	85	[1]	129	[1]	240	[2]	256	[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 $19 million decrease to RNF for the amortization of intangible assets and liabilities related to commodity contracts for the three months ended September 30, 2017, unrealized mark-to-market gains of $71 million and $73 million for the three months ended September 30, 2018 and 2017, respectively, accelerated nuclear fuel amortization associated with announced early plant retirements as discussed in Note 8 - Early Plant Retirements of $18 million and $6 million decrease to revenue net of purchased power and fuel expense for the three months ended September 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 $41 million decrease to RNF for the amortization of intangible assets and liabilities related to commodity contracts for the nine months ended September 30, 2017, unrealized mark-to-market losses of $104 million and $161 million for the nine months ended September 30, 2018 and 2017, respectively, accelerated nuclear fuel amortization associated with announced early plant retirements as discussed in Note 8 - Early Plant Retirements of $53 million and $8 million decrease to revenue net of purchased power and fuel expense for the nine months ended September 30, 2018 and 2017, 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.