Investments (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Variable Interest Entity
Number of consolidated variable interest entities	4
Carrying amount of consolidated assets and liabilities of VIE
Recourse debt relating to a group of solar entities	$ 130.0
Voting interest in RF Hold Co LLC (as a percent)		100.00%
Letters of credit
Variable Interest Entity
Credit facility provided to solar entity group	150
Consolidated Variable Interest Entities.
Variable Interest Entity
Number of consolidated variable interest entities	4
Ownership percentage, nonconsolidated variable interest entity (as a percent)	100.00%
Carrying amount of consolidated assets and liabilities of VIE
Current assets	481.5	516.6
Noncurrent assets	348.6	57.7
Total Assets	830.1	574.3
Current liabilities	483.4	345.5
Noncurrent liabilities	540.0	399.0
Total Liabilities	1,023.4	744.5
RSB BondCo. LLC
Variable Interest Entity
Remittance by BGE of payments received from customers for rate stabilization	92.3	90.3	85.8
Retail gas customer supply operation
Variable Interest Entity
New variable interest entities formed			2
Variable interest entity, ownership percentage (as a percent)	100.00%
Retail gas customer supply operation | Parental guarantee
Variable Interest Entity
Parental guarantee provided	$ 75
Unconsolidated Variable Interest Entities.
Variable Interest Entity
Number of Variable Interest Entities not consolidated by equity holders	6	6