Derivative Financial Instruments (Credit Risk) (Details) (USD $)	Sep. 30, 2012
Exelon Generation Co L L C [Member]
Credit Risk [Abstract]
Financial institutions	$ 386,000,000
Investor-owned utilities, marketers and power producers	902,000,000
Energy cooperative and municipalities	710,000,000
Other	72,000,000
Total	2,070,000,000
Due From Related Parties [Abstract]
Net receivable from electric utility	53,000,000
Net receivable from affiliated electric and gas utility	53,000,000
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	1,968,000,000
Non-investment grade	46,000,000
No external ratings - internally rated - investment grade	501,000,000
No external ratings - internally rated - non-investment grade	90,000,000
Total	2,605,000,000
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	492,000,000
Non-investment grade	25,000,000
No external ratings - internally rated - investment grade	16,000,000
No external ratings - internally rated - non-investment grade	2,000,000
Total	535,000,000
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	1,476,000,000
Non-investment grade	21,000,000
No external ratings - internally rated - investment grade	485,000,000
No external ratings - internally rated - non-investment grade	88,000,000
Total	2,070,000,000
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	0
Non-investment grade	0
No external ratings - internally rated - investment grade	1,000,000
No external ratings - internally rated - non-investment grade	0
Total	1,000,000
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	0
Non-investment grade	0
No external ratings - internally rated - investment grade	267,000,000
No external ratings - internally rated - non-investment grade	0
Total	267,000,000
PECO Energy Co [Member]
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Natural gas asset manager parental guarantee	14,000,000
Credit exposure under natural gas supply and management agreements	8,000,000
Baltimore Gas and Electric Company [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	11,000,000
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Credit exposure under off system sales	$ 1,000,000