Investment in Constellation Energy Nuclear Group, LLC (Details) (USD $)	1 Months Ended								3 Months Ended		1 Months Ended
	Mar. 31, 2012	May 31, 2011	Mar. 31, 2012 Minimum [Member]	Mar. 31, 2012 Maximum [Member]	Mar. 31, 2012 Power Purchase Agreement [Member]	Mar. 31, 2012 Power Service Agency Agreement [Member]	Mar. 31, 2012 Administrative Services Agreement [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member]	Mar. 31, 2011 Exelon Generation Co L L C [Member]	Mar. 31, 2012 Exelon Generation Co L L C [Member] Minimum [Member]	Mar. 31, 2012 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, 2012 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]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest in CENG (as a percent)	50.01%							50.01%	50.01%
CENG	$ (9,000,000)							$ (9,000,000)
Amortization of basis difference in CENG	(12,000,000)							(12,000,000)
Total equity investment earnings (losses) - CENG	(21,000,000)							(21,000,000)
Basis difference in investment in CENG	183,000,000
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%							85.00%	90.00%
Purchase of nuclear output by EDF (as a percent)	49.99%
Maximum cost of service per month	358,000
Impact Of Transactions Under Agreements [Abstract]
Increase (Decrease) in earnings					(35,000,000)	1,000,000	3,000,000		366,000,000	306,000,000			(35,000,000)	1,000,000	3,000,000
Amount receivable from (payable to) related to agreements with CENG					(60,000,000)	0	3,000,000						(60,000,000)	0	3,000,000
CENG issued an unsecured revolving promissory note to borrow up to an aggregate principal amount	$ 42,500,000	$ 62,500,000						$ 42,500,000
Percentage of request for borrowings submitted to entity (as a percent)	50.00%							50.00%
Percentage of request for borrowings submitted to entity's subsidiaries (as a percent)	50.00%							50.00%