Income Taxes (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 1999
Effective Income Tax Rate Reconciliation
U.S. Federal statutory rate	35.00%	[1]	35.00%
Increase (decrease) due to:
State income taxes, net of Federal income tax benefit	68.00%	[1]	(27.40%)
Qualified nuclear decommissioning trust fund income (losses)	62.00%	[1]	16.00%
Domestic production activities deduction	(2.40%)	[1]	(1.20%)
Tax exempt income	(1.60%)	[1]	(0.40%)
Health Care Reform Legislation	2.20%	[1]	0.20%
Amortization of investment tax credit	(25.80%)	[1]	(0.80%)
Plant basis differences	(24.90%)	[1]	(3.00%)
Production Tax Credits	(21.70%)	[1]	(0.50%)
Fines and Penalties			13.30%
Merger Expenses			13.20%
Other	7.40%	[1]	(0.30%)
Effective income tax rate	98.20%	[1]	44.10%
Accounting for Uncertainty in Income Taxes [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	$ 1,656,000,000					$ 1,024,000,000
1999 Sale of Fossil Generating Assets [Abstract]
Deferred tax gain on sale of fossil generating assets							2,800,000,000
Deferred tax gain under involuntary conversion provisions of the IRC							1,600,000,000
Deferred tax gain under like-kind exchange provisions of the IRC							1,200,000,000
IRS asserted penalties for understatement of tax	265,000,000
Potential tax and interest from a successful IRS challenge of the like-kind exchange transaction position	840,000,000
IRS asserted penalties for understatement of tax related to like-kind exchange	86,000,000
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Interest Expense				65,000,000
Tax Settlement [Abstract]
Payment to IRS for open tax positions					302,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	56,000,000		158,000,000
Exelon Generation Co L L C [Member]
Effective Income Tax Rate Reconciliation
U.S. Federal statutory rate	35.00%	[1]	35.00%
Increase (decrease) due to:
State income taxes, net of Federal income tax benefit	82.00%	[1]	1.20%
Qualified nuclear decommissioning trust fund income (losses)	(192.30%)	[1]	14.70%
Domestic production activities deduction	7.40%	[1]	(1.20%)
Tax exempt income	4.80%	[1]	(0.40%)
Amortization of investment tax credit	75.60%	[1]	(0.50%)
Production Tax Credits	67.20%	[1]	(3.10%)
Fines and Penalties			11.90%
Other	(74.10%)	[1]	0.50%
Effective income tax rate	5.60%	[1]	58.10%
Accounting for Uncertainty in Income Taxes [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	903,000,000					876,000,000
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Current Tax Expense (Benefit)				70,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	(1,000,000)		230,000,000
Commonwealth Edison Co [Member]
Effective Income Tax Rate Reconciliation
U.S. Federal statutory rate	35.00%		35.00%
Increase (decrease) due to:
State income taxes, net of Federal income tax benefit	5.80%		5.90%
Health Care Reform Legislation	(0.50%)		0.40%
Amortization of investment tax credit	0.40%		(0.30%)
Plant basis differences	0.90%
Other	0.10%		0.20%
Effective income tax rate	41.70%		41.20%
Accounting for Uncertainty in Income Taxes [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	334,000,000					67,000,000
1999 Sale of Fossil Generating Assets [Abstract]
IRS asserted penalties for understatement of tax	170,000,000
Potential tax and interest from a successful IRS challenge of the like-kind exchange transaction position	302,000,000
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Interest Expense				36,000,000
FIN 48 Tax Remeasurement Current Tax Expense (Benefit)				70,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	(58,000,000)		61,000,000
PECO Energy Co [Member]
Effective Income Tax Rate Reconciliation
U.S. Federal statutory rate	35.00%		35.00%
Increase (decrease) due to:
State income taxes, net of Federal income tax benefit	2.80%		3.40%
Amortization of investment tax credit	(0.10%)		(0.30%)
Plant basis differences	(6.70%)		(3.50%)
Other	0.10%		(0.10%)
Effective income tax rate	31.10%		34.50%
Accounting for Uncertainty in Income Taxes [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	44,000,000					44,000,000
Status Of Like Kind Exchange Position [Abstract]
Receivable from Exelon intercompany money pool	50,000,000					0
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Interest Expense				22,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	55,000,000		51,000,000
Baltimore Gas and Electric Company [Member]
Effective Income Tax Rate Reconciliation
U.S. Federal statutory rate	35.00%		35.00%
Increase (decrease) due to:
State income taxes, net of Federal income tax benefit	5.70%		4.40%
Health Care Reform Legislation	0.40%
Amortization of investment tax credit	(0.20%)		0.90%
Plant basis differences	(0.60%)		2.70%
Merger Expenses			(8.50%)
Other	0.40%		0.30%
Effective income tax rate	40.70%		34.80%
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	$ 55,000,000		$ (16,000,000)

[1] Generation and ComEd recognized a loss before income taxes for the three months ended March 31, 2013. As a result, positive percentages represent an income tax benefit for Generation and ComEd for the three months ended March 31, 2013. (b) Exelon activity for the three months ended March 31, 2012 includes the results of Constellation and BGE for March 12, 2012 - March 31, 2012. Generation activity for the three months ended March 31, 2012 includes the results of Constellation for March 12, 2012 - March 31, 2012.