Mergers, Acquisitions and Dispositions - Costs Recognized after Closing of Merger (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Jun. 30, 2012
Business Acquisition [Line Items]
BGE rate credit of $100 per residential customer	$ 113,000,000	[1]
Customer investment fund to invest in energy efficiency and low-income energy assistance to BGE customers	114,000,000
Contribution for renewable energy, energy efficiency or related projects in Baltimore	2,000,000
Charitable contributions at $7 million per year for 10 years	70,000,000
State funding for offshore wind development projects	32,000,000
Miscellaneous tax benefits	(2,000,000)
Total	329,000,000
Rate credit per residential customer (in usd per customer)	100
Charitable contributions, per year	7,000,000
Period for charitable contributions	10 years
Construction costs			66,000,000
Baltimore Gas and Electric Company [Member]
Business Acquisition [Line Items]
BGE rate credit of $100 per residential customer	113,000,000	[1]
Customer investment fund to invest in energy efficiency and low-income energy assistance to BGE customers	0
Contribution for renewable energy, energy efficiency or related projects in Baltimore	0
Charitable contributions at $7 million per year for 10 years	28,000,000
State funding for offshore wind development projects	0
Miscellaneous tax benefits	(2,000,000)
Total	139,000,000
Rate credit per residential customer (in usd per customer)	100
Charitable contributions, per year	7,000,000
Period for charitable contributions	10 years
Exelon Generation Co L L C [Member]
Business Acquisition [Line Items]
BGE rate credit of $100 per residential customer	0	[1]
Customer investment fund to invest in energy efficiency and low-income energy assistance to BGE customers	0
Contribution for renewable energy, energy efficiency or related projects in Baltimore	0
Charitable contributions at $7 million per year for 10 years	35,000,000
State funding for offshore wind development projects	0
Miscellaneous tax benefits	0
Total	35,000,000
Rate credit per residential customer (in usd per customer)	100
Charitable contributions, per year	$ 7,000,000
Period for charitable contributions	10 years

[1] Exelon made a $66 million equity contribution to BGE in the second quarter of 2012 to fund the after-tax amount of the rate credit as directed in the MDPSC order approving the merger transaction.