Income Taxes (Details) (USD $)	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Jun. 30, 2010	Dec. 31, 2010
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	(27.40%)	5.50%
Qualified nuclear decommissioning trust fund income (losses)	16.00%	2.30%
Domestic production activities deduction	(1.20%)	(0.90%)
Tax exempt income	(0.40%)	(0.10%)
Health Care Reform Legislation	0.20%
Amortization of investment tax credit	(0.80%)	(0.20%)
Plant basis differences	(3.00%)	0.00%
Fines and Penalties	13.30%
Merger Expenses	13.20%
Other	(0.30%)	(0.70%)
Production Tax Credits	(0.50%)	(0.90%)
Effective income tax rate	44.10%	40.00%
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
Potential tax and interest from a successful IRS challenge of the like-kind exchange transaction position	840,000,000
Potential interest expense from a successful IRS challenge of the like-kind exchange position	240,000,000
IRS withdrawn penalties originally asserted for understatement of tax related to involuntary conversion			110,000,000
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]
Tax and interest to be assessed under preliminary settlement with IRS within the next 12 months	300,000,000
Refund due from the settlement of the 2001 tax method of accounting change for certain overhead costs under SSCM	300,000,000
Payment to IRS for open tax positions				302,000,000
Income tax benefit recorded as a result of re-apportionment of state income taxes [Abstract]
Income tax benefit recorded as a result of re-apportionment of state income taxes	72,000,000
Gross Deferred Income tax asset recorded as a result of re-apportionment of state income taxes	116,000,000
Gross Deferred Income tax liability recorded as a result of re-apportionment of state income taxes	44,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	158,000,000	446,000,000
Exelon Generation Co L L C [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	1.20%	6.10%
Qualified nuclear decommissioning trust fund income (losses)	14.70%	3.20%
Domestic production activities deduction	(1.20%)	(1.30%)
Tax exempt income	(0.40%)	(0.10%)
Amortization of investment tax credit	(0.50%)	(0.20%)
Fines and Penalties	11.90%
Other	0.50%	(1.00%)
Production Tax Credits	(3.10%)	(1.30%)
Effective income tax rate	58.10%	40.40%
Unrecognized tax benefits that if recognized would affect the effective tax rate [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	874,000,000
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Current Tax Expense (Benefit)			70,000,000
Tax Settlement [Abstract]
Refunds expected to be received (paid) as a result of preliminary settlement with IRS	(10,000,000)
Income tax benefit recorded as a result of re-apportionment of state income taxes [Abstract]
Gross Deferred Income tax asset recorded as a result of re-apportionment of state income taxes	14,000,000
Gross Deferred Income tax liability recorded as a result of re-apportionment of state income taxes	14,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	230,000,000	336,000,000
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	5.90%	7.30%
Health Care Reform Legislation	0.40%
Amortization of investment tax credit	(0.30%)	(0.40%)
Other	0.20%	0.10%
Effective income tax rate	41.20%	42.00%
Unrecognized tax benefits that if recognized would affect the effective tax rate [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	69,000,000
1999 Sale of Fossil Generating Assets [Abstract]
Potential tax and interest from a successful IRS challenge of the like-kind exchange transaction position	520,000,000
Potential interest expense from a successful IRS challenge of the like-kind exchange position	180,000,000
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)
Tax Settlement [Abstract]
Refunds expected to be received (paid) as a result of preliminary settlement with IRS	(405,000,000)
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	61,000,000	50,000,000
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	3.40%	(3.90%)
Amortization of investment tax credit	(0.30%)	(0.30%)
Plant basis differences	(3.50%)	(0.20%)
Other	(0.10%)	0.20%
Effective income tax rate	34.50%	30.80%
Unrecognized tax benefits that if recognized would affect the effective tax rate [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	48,000,000
FIN 48 Tax Remeasurement [Abstract]
FIN 48 Tax Remeasurement Interest Expense			22,000,000
Tax Settlement [Abstract]
Refunds expected to be received (paid) as a result of preliminary settlement with IRS	135,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	51,000,000	56,000,000
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	4.40%	4.60%
Health Care Reform Legislation		(1.90%)
Amortization of investment tax credit	0.90%	(0.20%)
Plant basis differences	2.70%	(0.80%)
Merger Expenses	(8.50%)
Other	0.30%
Effective income tax rate	34.80%	36.70%
Unrecognized tax benefits that if recognized would affect the effective tax rate [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	11,000,000
Interest recognized related to uncertain tax positions [Abstract]
Net interest (income) expense recognized related to uncertain tax positions	5,000,000
Income Tax Expense Benefit Continuing Operations [Line Items]
Income Tax Expense (Benefit)	(16,000,000)	47,000,000
Exelon Corporate [Member]
Unrecognized tax benefits that if recognized would affect the effective tax rate [Abstract]
Unrecognized tax benefits that if recognized would affect the effective tax rate	$ 1,040,000,000