Basis of Presentation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Depreciation and amortization	$ 543	$ 382
Exelon Generation Co L L C [Member]
Depreciation and amortization	214	153
Commonwealth Edison Co [Member]
Depreciation and amortization	167	149
PECO Energy Co [Member]
Depreciation and amortization	57	53
Baltimore Gas and Electric Company [Member]
Depreciation and amortization	93	79
Baltimore Gas and Electric Company [Member] | Scenario Adjustment [Member]
Depreciation and amortization	$ 2