Stock-Based Compensation Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended							7 Months Ended		3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Performance Share Awards [Member]	Dec. 31, 2012 Performance Share Awards [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member]	Mar. 31, 2013 Commonwealth Edison Co [Member]	Mar. 31, 2012 Commonwealth Edison Co [Member]	Mar. 31, 2013 PECO Energy Co [Member]	Mar. 31, 2012 PECO Energy Co [Member]	Mar. 31, 2012 Baltimore Gas and Electric Company [Member]	Sep. 30, 2012 Baltimore Gas and Electric Company [Member]		Mar. 31, 2013 Business Services Company [Member]		Mar. 31, 2012 Business Services Company [Member]
Common Stock [Abstract]
Common Stock without par - Authorized	2,000,000,000		2,000,000,000
Common Stock without par - Outstanding	855,849,302		855,000,000
Authorized Shares for LTIP [Abstract]
Authorized Shares for LTIP	16,000,000
Share Based Compensation Components [Abstract]
Performance shares	$ 16	$ 16
Stock options	1	7
Restricted stock units	20	19
Other stock-based awards	1	1
Total stock-based compensation included in operating and maintenance expense	38	43
Income Tax Benefit	(15)	(16)
Total after-tax stock based compensation expense	23	27
Share Based Compensation Pre Tax Expense [Line Items]
Pre-tax stock based compensation expense	38	43				15	14	2	5	2	3	2	2	[1]	17	[2],[3]	19	[2],[3]
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward
Expected Payout Minimum				50.00%	75.00%
Expected Payout Maximum				150.00%	125.00%
Additional Information Regarding Stock Options Exercised [Abstract]
Unrecognized compensation costs related to nonvested stock options	5
Weighted Average Period Non Vested Stock Options Are Expected To Be Recognized Over	2 years 1 month 7 days
Restricted Stock Units [Abstract]
Obligations related to outstanding restricted stock units not yet settled	48
Obligations related to outstanding restricted stock units that will be settled in cash	0
Fair value of settled restricted stock	22	20
Unrecognized compensation costs related to nonvested restricted stock units	94
Weighted average period	2 years 7 months 9 days
Performance Share Awards [Abstract]
Percentage of current performance share awards settled in stock	50.00%
Percentage of current performance share awards settled in cash	50.00%
Percentage of current performance share awards settled in cash for executive vice presidents and higher officers	100.00%
Expected TSR Percentage Increase	25.00%
Expected TSR Percentage Decrease	25.00%
Expected IPM Percentage Decrease	50.00%
Expected IPM Percentage Increase	20.00%
Expected IPM Percentage Increase for Senior VPs and above	10.00%
Performance shares settled at fair value of	22	18
Performance shares settled at fair value and paid with cash	9	3
Unrecognized compensation costs related to nonvested performance shares	47
Weighted average period	2 years 8 months 16 days
Performance Share Transition Awards [Abstract]
Unrecognized compensation costs related to nonvested performance shares transition	$ 29
Weighted average period	1 year 9 months 22 days

[1] BGE's stock-based compensation expense (pre-tax) for the three months ended March 31, 2012 includes $2 million of cost incurred prior to the closing of Exelon's merger with Constellation on March 12, 2012. This amount is not included in Exelon's stock-based compensation expense for the three months ended March 31, 2012 shown in the table titled Components of Stock-Based Compensation Expense above.

[2] These amounts primarily represent amounts billed to Exelon’s subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO and BGE amounts above. (c)        The stock-based compensation expense (pre-tax) for the three months ended March 31, 2013 reflects the impact of changes to the retirement eligibility requirements for employees participating in the LTIP. In addition, the stock-based compensation expense at ComEd reflects the adoption of the ComEd Key Manager Long-Term Performance Program in 2013 for certain employees, which is not consider stock-based compensation expense under the applicable authoritative guidance. In 2012, these employees participated in the Exelon Restricted Stock Award Program.

[3] These amounts primarily represent amounts billed to Exelon's subsidiaries through intercompany allocations. These amounts are not included in the Generation, ComEd, PECO and BGE amounts above.