Derivative Financial Instruments (Credit Risk) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Exelon Generation Co L L C [Member]
Credit Risk [Abstract]
Financial institutions	$ 422.00
Investor-owned utilities, marketers and power producers	865.00
Energy cooperative and municipalities	786.00
Other	108.00
Total	2,181.00
Due From Related Parties [Abstract]
Net receivable from electric utility	54.00
Net receivable from affiliated electric and gas utility	56.00
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	1,984.00
Non-investment grade	28.00
No external ratings - internally rated - investment grade	512.00
No external ratings - internally rated - non-investment grade	41.00
Total	2,565.00
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	347.00	[1]
Non-investment grade	24.00	[1]
No external ratings - internally rated - investment grade	10.00	[1]
No external ratings - internally rated - non-investment grade	3.00	[1]
Total	384.00	[1]
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	1,637.00
Non-investment grade	4.00
No external ratings - internally rated - investment grade	502.00
No external ratings - internally rated - non-investment grade	38.00
Total	2,181.00
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	1.00
No external ratings - internally rated - investment grade	1.00
Total	2.00
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	262.00
No external ratings - internally rated - investment grade	271.00
Total	533.00
Commonwealth Edison Co [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	54.00
PECO Energy Co [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	56.00
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Natural gas asset manager parental guarantee	20.00
Credit exposure under natural gas supply and management agreements	7.00
Baltimore Gas and Electric Company [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	31.00
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Credit exposure under off system sales	$ 8.00

[1] As of December 31, 2012, credit collateral held from counterparties where Generation had credit exposure included $344 million of cash and $40 million of letters of credit .