Segment Information - Generation Total Revenues Net of Purchased Power and Fuel Expense (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Amortization Of Intangible Assets Related To Commodity Contracts For Revenue Net Purchased Power And Fuel	$ 124		$ 488		$ 1,098
Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	3,417		3,270		3,433
Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	2,594		2,586		2,998
Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	351		185		196
Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	483		(4)		76
Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	317		436		405
Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	327	[1]	201	[1]	131	[1]
Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	7,489		6,674		7,239
Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	(21)	[2]	759	[2]	137	[2]
Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense, Total	7,468		7,433		7,376
Operating Segments [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	3,466	[3]	3,273	[3]	3,477	[3]
Operating Segments [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	2,580	[3]	2,585	[3]	2,974	[3]
Operating Segments [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	432	[3]	217	[3]	151	[3]
Operating Segments [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	457	[3]	14	[3]	101	[3]
Operating Segments [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	573	[3]	604	[3]	403	[3]
Operating Segments [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	611	[1],[3]	334	[1],[3]	53	[1],[3]
Operating Segments [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	8,119	[3]	7,027	[3]	7,159	[3]
Operating Segments [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	(651)	[2],[3]	406	[2],[3]	217	[2],[3]
Operating Segments [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from external customers	7,468	[3]	7,433	[3]	7,376	[3]
Intersegment Eliminations [Member] | Generation Mid Atlantic [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	(49)		(3)		(44)
Intersegment Eliminations [Member] | Generation Midwest [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	14		1		24
Intersegment Eliminations [Member] | Generation New England [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	(81)		(32)		45
Intersegment Eliminations [Member] | Generation New York [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	26		(18)		(25)
Intersegment Eliminations [Member] | Generation ERCOT [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	(256)		(168)		2
Intersegment Eliminations [Member] | Generation Other Regions [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	(284)	[1]	(133)	[1]	78	[1]
Intersegment Eliminations [Member] | Generation Reportable Segments Total [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	(630)		(353)		80
Intersegment Eliminations [Member] | Generation All Other Segments [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	630	[2]	353	[2]	(80)	[2]
Intersegment Eliminations [Member] | Generation Total Consolidated Group [Member]
Segment Reporting Information [Line Items]
Revenue net of purchased power and fuel expense from transactions with other operating segments of the same entity	$ 0		$ 0		$ 0

[1] Other regions include the South, West and Canada, which are not considered individually significant.

[2] Other represents activities not allocated to a region. See text above for a description of included activities. Also includes amortization of intangible assets related to commodity contracts recorded at fair value of $124 million, $488 million, and $1,098 million, for the years ended December 31, 2014, 2013, and 2012, respectively, and the elimination of intersegment revenue net of purchased power and fuel expense.

[3] Includes purchases and sales from third parties and affiliated sales to ComEd, PECO and BGE.