Merger and Acquisitions (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Finite Lived Intangible Assets Net [Abstract]
Amortization of Regulatory Asset	$ 19,000,000		$ 9,000,000
Business Combination, Bargain Purchase [Abstract]
Business Combination, Bargain Purchase, Gain Recognized, Amount			0
Business Acquisition, Impact Of Merger [Abstract]
Revenues	6,082,000,000		4,690,000,000
Net income	1,000,000		200,000,000
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Construction Time Frame	2 years
Business Acquisition, Gain (Loss) On Assets Sold	272,000,000
Business Acquisition, Gain (Loss) On Assets Sold Update	8,000,000
Business Acquisition Potential Cash Payment	40,000,000
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Period Start	111,000,000
Severance Charges	2,000,000		67,000,000
Stock Compensation Expense	1,000,000		8,000,000
Other Expense Charges			8,000,000
Total Severance Benefits	3,000,000	[1]	83,000,000	[1]
Payments	22,000,000
Restructuring Reserve, Period End	92,000,000		83,000,000
Severance [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation	2,000,000	[2]	67,000,000	[2]
One-time Termination Benefits [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation	1,000,000
Stock Compensation Plan [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation	1,000,000		8,000,000
Other Severance Charges [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation			8,000,000	[3]
Constellation Energy Group Acquisition [Member]
Restructuring Reserve [Roll Forward]
Merger related regulatory asset	6,000,000		32,000,000
Minimum [Member]
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Construction Cost	95,000,000
Maximum [Member]
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Construction Cost	120,000,000
Constellation Energy Group Acquisition [Member]
Business Acquisition Cost Of Acquired Entity [Abstract]
Business Combination, Integration Related Costs	33,000,000		516,000,000
Business Acquisition, Purchase Price Allocation [Abstract]
Business Acquisition, Purchase Price Allocation, Current Assets	4,936,000,000
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	9,342,000,000
Business Acquisition, Purchase Price Allocation, Intangible Assets, Contracts	3,218,000,000
Business Acquisition, Purchase Price Allocation, Intangible Assets, Relationships	457,000,000
Business Acquisition, Purchase Price Allocation, CENG Investment	1,942,000,000
Business Acquisition, Purchase Price Allocation, Pension Regulatory Asset	740,000,000
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets	2,265,000,000
Business Acquisition, Purchase Price Allocation, Assets Acquired	22,900,000,000
Business Acquisition, Purchase Price Allocation, Current Liabilities	3,408,000,000
Business Acquisition, Purchase Price Allocation, Other Noncurrent Liabilities	4,683,000,000
Business Acquisition, Purchase Price Allocation, Liability Contracts	1,722,000,000
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt	5,632,000,000
Business Combination Acquisition Of Less Than 100 Percent Noncontrolling Interest Fair Value	90,000,000
Business Acquisition, Purchase Price Allocation, Liabilities Assumed	15,535,000,000
Business Acquisition, Purchase Price Allocation, Assets Acquired (Liabilities Assumed), Net	7,365,000,000
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue			6,977,000,000
Business Acquisition, Pro Forma Net Income (Loss)			409,000,000
Business Acquisition, Pro Forma Earnings Per Share, Basic			$ 0.48
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Direct Investment With State And Local Governments Due To Settlement	1,000,000,000
Business Acquisition, Development Of New Generation Cost	625,000,000
Business Acquisition, Expected New Generation Development Time Frame	10 years
Constellation Energy Group Acquisition [Member] | Minimum [Member]
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Expected New Generation Mwh	285
Constellation Energy Group Acquisition [Member] | Maximum [Member]
Business Acquisition, Regulatory Matters [Abstract]
Business Acquisition, Expected New Generation Mwh	300
Exelon Generation Co L L C [Member]
Business Combination, Bargain Purchase [Abstract]
Business Combination, Bargain Purchase, Gain Recognized, Amount			0
Business Acquisition, Impact Of Merger [Abstract]
Revenues	3,533,000,000		2,743,000,000
Net income	(17,000,000)		166,000,000
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Period Start	33,000,000
Severance Charges	2,000,000		35,000,000
Stock Compensation Expense	1,000,000		5,000,000
Other Expense Charges			5,000,000
Total Severance Benefits	3,000,000	[1]	45,000,000	[1]
Payments	8,000,000
Restructuring Reserve, Intercompany Allocation	3,000,000		29,000,000
Restructuring Reserve, Period End	25,000,000		16,000,000
Exelon Generation Co L L C [Member] | Severance [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation			13,000,000	[2]
Exelon Generation Co L L C [Member] | Stock Compensation Plan [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation			2,000,000
Exelon Generation Co L L C [Member] | Other Severance Charges [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation			1,000,000	[3]
Exelon Generation Co L L C [Member] | Constellation Energy Group Acquisition [Member]
Business Acquisition Cost Of Acquired Entity [Abstract]
Business Combination, Integration Related Costs	23,000,000		110,000,000
Business Acquisition, Purchase Price Allocation [Abstract]
Business Acquisition, Purchase Price Allocation, Current Assets	3,638,000,000
Business Acquisition, Purchase Price Allocation, Property, Plant and Equipment	4,054,000,000
Business Acquisition, Purchase Price Allocation, Intangible Assets, Contracts	3,218,000,000
Business Acquisition, Purchase Price Allocation, Intangible Assets, Relationships	457,000,000
Business Acquisition, Purchase Price Allocation, CENG Investment	1,942,000,000
Business Acquisition, Purchase Price Allocation, Other Noncurrent Assets	1,266,000,000
Business Acquisition, Purchase Price Allocation, Assets Acquired	14,575,000,000
Business Acquisition, Purchase Price Allocation, Current Liabilities	2,804,000,000
Business Acquisition, Purchase Price Allocation, Other Noncurrent Liabilities	1,933,000,000
Business Acquisition, Purchase Price Allocation, Liability Contracts	1,512,000,000
Business Acquisition, Purchase Price Allocation, Noncurrent Liabilities, Long-term Debt	2,972,000,000
Business Combination Acquisition Of Less Than 100 Percent Noncontrolling Interest Fair Value	90,000,000
Business Acquisition, Purchase Price Allocation, Liabilities Assumed	9,311,000,000
Business Acquisition, Purchase Price Allocation, Assets Acquired (Liabilities Assumed), Net	5,264,000,000
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue			3,997,000,000
Business Acquisition, Pro Forma Net Income (Loss)			129,000,000
Commonwealth Edison Co [Member]
Business Acquisition, Impact Of Merger [Abstract]
Revenues	1,160,000,000		1,388,000,000
Net income	(81,000,000)		87,000,000
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Period Start	1,000,000
Severance Charges			9,000,000	[4]
Stock Compensation Expense			1,000,000	[4]
Other Expense Charges			1,000,000	[4]
Total Severance Benefits			11,000,000	[1],[4]
Restructuring Reserve, Intercompany Allocation			11,000,000
Restructuring Reserve, Period End	1,000,000
Merger related severance regulatory asset			11,000,000
Commonwealth Edison Co [Member] | Constellation Energy Group Acquisition [Member]
Restructuring Reserve [Roll Forward]
Merger related regulatory asset	4,000,000		16,000,000
Commonwealth Edison Co [Member] | Constellation Energy Group Acquisition [Member]
Business Acquisition Cost Of Acquired Entity [Abstract]
Business Combination, Integration Related Costs	4,000,000		18,000,000
PECO Energy Co [Member]
Business Acquisition, Impact Of Merger [Abstract]
Revenues	895,000,000		875,000,000
Net income	122,000,000		97,000,000
Restructuring Reserve [Roll Forward]
Severance Charges			3,000,000
Stock Compensation Expense			1,000,000
Other Expense Charges			1,000,000
Total Severance Benefits			5,000,000	[1]
Restructuring Reserve, Intercompany Allocation			5,000,000
PECO Energy Co [Member] | Constellation Energy Group Acquisition [Member]
Business Acquisition Cost Of Acquired Entity [Abstract]
Business Combination, Integration Related Costs	3,000,000		7,000,000
Baltimore Gas and Electric Company [Member]
Business Acquisition, Impact Of Merger [Abstract]
Revenues	880,000,000		697,000,000
Revenues Impact	880,000,000		52,000,000
Net Income Loss Impact	80,000,000		(65,000,000)
Net income	80,000,000		(30,000,000)
Regulatory Assets Transfer Changes	8,000,000
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Period Start	11,000,000
Severance Charges			14,000,000	[5]
Stock Compensation Expense			1,000,000	[5]
Other Expense Charges			1,000,000	[5]
Total Severance Benefits			16,000,000	[1],[5]
Payments	2,000,000
Restructuring Reserve, Intercompany Allocation			5,000,000
Restructuring Reserve, Period End	9,000,000		11,000,000
Merger related severance regulatory asset			16,000,000
Baltimore Gas and Electric Company [Member] | Severance [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Intercompany Allocation			11,000,000
Baltimore Gas and Electric Company [Member] | Constellation Energy Group Acquisition [Member]
Restructuring Reserve [Roll Forward]
Merger related regulatory asset	2,000,000		16,000,000
Baltimore Gas and Electric Company [Member] | Constellation Energy Group Acquisition [Member]
Business Acquisition Cost Of Acquired Entity [Abstract]
Business Combination, Integration Related Costs	2,000,000		169,000,000
Business Acquisition, Impact Of Merger [Abstract]
Regulatory Assets Transfer Changes	6,000,000
Constellation Energy Group Acquisition [Member] | Total Assets [Member]
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross	1,953,000,000
Finite lived intangible assets accumulated amortization	(1,179,000,000)
Finite lived intangible assets net	774,000,000
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future amortization expense within the next twelve months	276,000,000
Future amortization expense in year two	116,000,000
Future amortizationExpense in year three	62,000,000
Future amortization expense in year four	12,000,000
Future amortization expense in year five	21,000,000
Future amortization expense in year six and thereafter	287,000,000
Customer Relationships [Member]
Finite Lived Intangible Assets Net [Abstract]
Amortization Of Contracts	5,000,000		1,000,000
Customer Relationships [Member] | Constellation Energy Group Acquisition [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Asset Weighted Average Useful Life	12 years 4 months 26 days
Customer Relationships [Member] | Constellation Energy Group Acquisition [Member] | Total Assets [Member]
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross	214,000,000
Finite lived intangible assets accumulated amortization	(20,000,000)
Finite lived intangible assets net	194,000,000
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future amortization expense within the next twelve months	15,000,000
Future amortization expense in year two	19,000,000
Future amortizationExpense in year three	19,000,000
Future amortization expense in year four	19,000,000
Future amortization expense in year five	19,000,000
Future amortization expense in year six and thereafter	103,000,000
Trade Names [Member]
Finite Lived Intangible Assets Net [Abstract]
Amortization Of Contracts	6,000,000		2,000,000
Trade Names [Member] | Constellation Energy Group Acquisition [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Asset Weighted Average Useful Life	10 years
Trade Names [Member] | Constellation Energy Group Acquisition [Member] | Total Assets [Member]
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross	243,000,000
Finite lived intangible assets accumulated amortization	(26,000,000)
Finite lived intangible assets net	217,000,000
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future amortization expense within the next twelve months	18,000,000
Future amortization expense in year two	24,000,000
Future amortizationExpense in year three	24,000,000
Future amortization expense in year four	24,000,000
Future amortization expense in year five	24,000,000
Future amortization expense in year six and thereafter	103,000,000
Power Supply Contracts [Member]
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets net	70,000,000
Amortization Of Contracts	170,000,000		131,000,000
Power Supply Contracts [Member] | Constellation Energy Group Acquisition [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Acquired Finite Lived Intangible Asset Weighted Average Useful Life	1 year 6 months 3 days
Power Supply Contracts [Member] | Constellation Energy Group Acquisition [Member] | Total Assets [Member]
Finite Lived Intangible Assets Net [Abstract]
Finite lived intangible assets gross	1,496,000,000	[6]
Finite lived intangible assets accumulated amortization	(1,133,000,000)	[6]
Finite lived intangible assets net	363,000,000	[6]
Finite Lived Intangible Assets Future Amortization Expense [Abstract]
Future amortization expense within the next twelve months	243,000,000	[6]
Future amortization expense in year two	73,000,000	[6]
Future amortizationExpense in year three	19,000,000	[6]
Future amortization expense in year four	(31,000,000)	[6]
Future amortization expense in year five	(22,000,000)	[6]
Future amortization expense in year six and thereafter	$ 81,000,000	[6]

[1] The amounts above include $3 million and $29 million at Generation, $0 million and $11 million at ComEd, $0 million and $5 million at PECO, and $0 million and $5 million at BGE, for amounts billed by BSC through intercompany allocations for the three months ended March 31, 2013 and 2012, respectively.

[2] Includes salary continuance and health and welfare severance benefits. Amounts represent ongoing severance plan benefits. Amounts also include one-time termination benefits of $1 million and $0 million for Exelon and Generation, respectively, as of March 31, 2013, which they began to recognize in the second quarter of 2012.

[3] Primarily includes life insurance, employer payroll taxes, educational assistance, and outplacement services.

[4] ComEd established regulatory assets of $0 million and $11 million for severance benefits costs for the three months ended March 31, 2013 and 2012, respectively. The majority of these costs are expected to be recovered over a five-year period.

[5] BGE established regulatory assets of $0 million and $16 million for severance benefits costs for the three months ended March 31, 2013 and 2012, respectively. The majority of these costs are being recovered over a five-year period beginning in March 2013.

[6] Includes the fair value of BGE's power and gas supply contracts of $70 million for which an offsetting regulatory asset was also recorded