Income Taxes, Unrecognized Tax Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Mar. 31, 2010 year	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Taxes Disclosure
Effective income tax rate (as a percent)	34.30%	40.80%		37.90%	42.50%
Unrecognized tax benefits
Total unrecognized tax benefits, beginning balance				$ 239.8
Increases in tax positions related to the current year				0.9
Increases in tax positions related to prior years				29.7
Reductions in tax positions related to prior years				(89.1)
Total unrecognized tax benefits, ending balance	181.3			181.3
Amount of potential decrease in income tax expense if unrecognized tax benefits were realized	181.3			181.3
Amount of decrease in income tax expense if unrecognized tax benefits from disallowed state tax refund claims subject to appeals were realized	170			170
State tax refund claims	55.9			55.9
Expected decrease in unrecognized tax benefits within the next year	12			12
Interest and penalties recorded as tax (benefit) expense	(1.6)	3.0		3.9	(4.3)
Accrued interest and penalties recognized in balance sheet	20.7			20.7		16.8
Baltimore Gas and Electric Company
Taxes Disclosure
Effective income tax rate (as a percent)	176.20%	34.00%		35.10%	39.60%
Small pre- tax loss as a result of Hurricane Irene storm-related expenses	2.1
Electric regulatory asset, amortization period (in years)			5
Unrecognized tax benefits
Total unrecognized tax benefits, ending balance	12.0			12.0
Accrued interest and penalties recognized in balance sheet	$ 1.1			$ 1.1		$ 3.8