Intangible Assets - Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Goodwill [Line Items]
Amortization of Intangible Assets	$ 179	[1]	$ 478	[1]	$ 1,150	[1]
Finite lived intangible assets gross	2,489
Commonwealth Edison Co [Member]
Goodwill [Line Items]
Amortization of Intangible Assets	7		7		7
Current alternative or renewable energy credits	4		3
Exelon Generation Co L L C [Member]
Goodwill [Line Items]
Amortization of Intangible Assets	179	[1]	550	[1]	1,145	[1]
Current alternative or renewable energy credits	191		158
Noncurrent alternative or renewable energy credits	44
PECO Energy Co [Member]
Goodwill [Line Items]
Amortization of Intangible Assets	0	[2]	0	[2]	0	[2]
Current alternative or renewable energy credits	13		19
Noncurrent alternative or renewable energy credits			5
Unamortized Energy Contracts [Member]
Goodwill [Line Items]
Amortization of Intangible Assets	135	[3],[4]	430	[3],[4]	1,110	[3],[4]
Unamortized Energy Contracts [Member] | Exelon Generation Co L L C [Member]
Goodwill [Line Items]
Amortization of Intangible Assets	135	[3],[4]	507	[3],[4]	1,110	[2]
Constellation Energy Group LLC [Member] | Unamortized Energy Contracts [Member]
Goodwill [Line Items]
Finite lived intangible assets gross	1,499
Constellation Energy Group LLC [Member] | Trade Names [Member]
Goodwill [Line Items]
Useful life	10 years
Finite lived intangible assets gross	$ 243

[1] At Exelon, amortization of unamortized energy contracts totaling $135 million, $430 million and $1,110 million for the years ended December 31, 2014, 2013 and 2012, respectively, was recorded in Purchase power and fuel expense or Operating revenues within Exelon’s Consolidated Statement of Operations and Comprehensive Income. At Generation, amortization of unamortized energy contracts totaling $135 million, $507 million and $1,110 million for the years ended December 31, 2014, 2013 and 2012, respectively, was recorded in Purchase power and fuel expense or Operating revenues within Generation’s Consolidated Statement of Operations and Comprehensive Income

[2] Included in Operating revenues or Purchased power and fuel on the Registrants’ Consolidated Statements of Operations and Comprehensive Income.

[3] Includes unamortized energy contract assets and liabilities on Exelon's and Generation's Consolidated Balance Sheets. Excludes $26 million of other miscellaneous unamortized energy contracts that have been acquired at various points in time. The estimated amortization for these miscellaneous unamortized energy contracts is $4 million, $3 million, $0 million, $2 million and $2 million for 2015, 2016, 2017, 2018 and 2019, respectively.

[4] In March 1999, ComEd entered into a settlement agreement with the City of Chicago associated with ComEd’s franchise agreement. Under the terms of the settlement, ComEd agreed to make payments to the City of Chicago each year from 1999 to 2002. The intangible asset recognized as a result of these payments is being amortized ratably over the remaining term of the franchise agreement, which ends in 2020.