Investment in Constellation Energy Nuclear Group, LLC (Details) (USD $)	3 Months Ended				1 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended							1 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Mar. 31, 2012 Power Purchase Agreement [Member]	Mar. 31, 2013 Power Purchase Agreement [Member]	Mar. 31, 2012 Power Service Agency Agreement [Member]	Mar. 31, 2013 Power Service Agency Agreement [Member]	Mar. 31, 2012 Administrative Services Agreement [Member]	Mar. 31, 2013 Administrative Services Agreement [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member]		Mar. 31, 2012 Exelon Generation Co L L C [Member]		Mar. 31, 2013 Exelon Generation Co L L C [Member] Minimum [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Maximum [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] Power Purchase Agreement [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Power Purchase Agreement [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] Power Service Agency Agreement [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Power Service Agency Agreement [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] Administrative Services Agreement [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] Administrative Services Agreement [Member]	Mar. 31, 2013 Exelon Generation Co L L C [Member] CENG [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] CENG [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in CENG (as a percent)											50.01%
CENG																							$ 15,000,000	$ (9,000,000)
Amortization of basis difference in CENG																							(27,000,000)	(12,000,000)
Total equity investment earnings (losses) - CENG																							(12,000,000)	(21,000,000)
Basis difference in investment in CENG											204,000,000
Related Party Transaction [Line Items]
Required purchases of power from CENG's nuclear plants not sold to third parties (as a percent)															85.00%	90.00%
Purchase of nuclear output by EDF (as a percent)											49.99%
Maximum cost of service per month											280,000
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings					(35,000,000)	(248,000,000)	1,000,000	1,000,000	3,000,000	11,000,000	392,000,000		370,000,000				(35,000,000)	(248,000,000)	1,000,000	1,000,000	3,000,000	11,000,000
Amount receivable from (payable to) related to agreements with CENG						(70,000,000)		0		4,000,000								(70,000,000)		0		4,000,000
Amortization of energy contract assets and liabilities	$ 176,000,000	[1]	$ 131,000,000	[1]							$ 176,000,000	[1]	$ 131,000,000	[1]

[1] Included in revenues or fuel expense on the Registrants' Consolidated Statements of Operations and Comprehensive Income.