Fair Value of Financial Assets and Liabilities (Fair Value By Balance Sheet Grouping) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying Reported Amount Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	$ 444	$ 214
Long-term debt (including amounts due within one year)	18,871	18,745
Long-term debt to financing trusts	648	648
Spent nuclear fuel obligation	1,020	1,020
Preferred securities of subsidiary	87	87
Estimate Of Fair Value Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	444	214
Long-term debt (including amounts due within one year)	20,622	20,520
Long-term debt to financing trusts	662	664
Spent nuclear fuel obligation	794	763
Preferred securities of subsidiary	93	82
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 1 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	1	4
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	443	210
Long-term debt (including amounts due within one year)	20,200	20,244
Spent nuclear fuel obligation	794	763
Preferred securities of subsidiary	93	82
Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	422	276
Long-term debt to financing trusts	662	664
Exelon Generation Co L L C [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	13
Long-term debt (including amounts due within one year)	7,621	7,483
Spent nuclear fuel obligation	1,020	1,020
Exelon Generation Co L L C [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	13
Long-term debt (including amounts due within one year)	8,026	7,849
Spent nuclear fuel obligation	794	763
Exelon Generation Co L L C [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	13
Long-term debt (including amounts due within one year)	7,621	7,591
Spent nuclear fuel obligation	794	763
Exelon Generation Co L L C [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	405	258
Commonwealth Edison Co [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	220
Long-term debt (including amounts due within one year)	5,568	5,567
Long-term debt to financing trusts	206	206
Commonwealth Edison Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	220
Long-term debt (including amounts due within one year)	6,503	6,548
Long-term debt to financing trusts	213	212
Commonwealth Edison Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	220
Long-term debt (including amounts due within one year)	6,485	6,530
Long-term debt to financing trusts	0
Commonwealth Edison Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	18	18
Long-term debt to financing trusts	213	212
PECO Energy Co [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	210	210
Long-term debt (including amounts due within one year)	1,948	1,947
Long-term debt to financing trusts	184	184
Preferred securities of subsidiary	87	87
PECO Energy Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	210	210
Long-term debt (including amounts due within one year)	2,242	2,264
Long-term debt to financing trusts	188	188
Preferred securities of subsidiary	93	82
PECO Energy Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Short-term liabilities	210	210
Long-term debt (including amounts due within one year)	2,242	2,264
Preferred securities of subsidiary	93	82
PECO Energy Co [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt to financing trusts	188	188
Baltimore Gas and Electric Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	2,178	2,178
Long-term debt to financing trusts	258	258
Baltimore Gas and Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	2,464	2,468
Long-term debt to financing trusts	261	263
Baltimore Gas and Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 2 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt (including amounts due within one year)	2,464	2,468
Baltimore Gas and Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] | Fair Value Inputs Level 3 [Member]
Financial Instruments Financial Liabilities Balance Sheet Groupings [Abstract]
Long-term debt to financing trusts	$ 261	$ 263