Derivative Financial Instruments (Credit Risk) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Exelon Generation Co L L C [Member]
Credit Risk [Abstract]
Financial institutions	$ 352
Investor-owned utilities, marketers and power producers	515
Energy cooperative and municipalities	824
Other	45
Total	1,736
Exelon Generation Co L L C [Member] | Commonwealth Edison Co Affiliate [Member]
Due From Related Parties [Abstract]
Net receivable from electric utility	44
Exelon Generation Co L L C [Member] | PECO Energy Co Affiliate [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	59
Exelon Generation Co L L C [Member] | Baltimore Gas And Electric Company Affiliate [Member]
Due From Related Parties [Abstract]
Net receivable from affiliated electric and gas utility	25
Exelon Generation Co L L C [Member] | Total Exposure Before Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	1,459
Non-investment grade	49
No external ratings - internally rated - investment grade	403
No external ratings - internally rated - non-investment grade	44
Total	1,955
Exelon Generation Co L L C [Member] | Credit Collateral [Member]
Credit Risk [Abstract]
Investment grade	184
Non-investment grade	29
No external ratings - internally rated - investment grade	5
No external ratings - internally rated - non-investment grade	1
Total	219
Exelon Generation Co L L C [Member] | Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	1,275
Non-investment grade	20
No external ratings - internally rated - investment grade	398
No external ratings - internally rated - non-investment grade	43
Total	1,736
Exelon Generation Co L L C [Member] | Number Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	1
Non-investment grade	0
No external ratings - internally rated - investment grade	1
No external ratings - internally rated - non-investment grade	0
Total	2
Exelon Generation Co L L C [Member] | Net Exposure Of Counterparties Greater Than Ten Percent Of Net Exposure [Member]
Credit Risk [Abstract]
Investment grade	387
Non-investment grade	0
No external ratings - internally rated - investment grade	252
No external ratings - internally rated - non-investment grade	0
Total	639
PECO Energy Co [Member]
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Credit exposure under natural gas supply and management agreements	0
Baltimore Gas and Electric Company [Member]
Natural Gas Supply And Management Agreement Credit Exposure [Abstract]
Credit exposure under off system sales	$ 8