Derivatives and Fair Value Measurements (Details 8) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Carrying Amount
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Investments and other assets	$ 146.8	$ 135.7
Fixed-rate long-term debt	4,132.5	4,229.3
Variable-rate long-term debt	892.3	528.7
Carrying Amount | Baltimore Gas and Electric Company
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Fixed-rate long-term debt	2,361.9	2,143.6
Fair Value
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Investments and other assets	146.8	136.2
Fixed-rate long-term debt	4,702.8	4,518.4
Variable-rate long-term debt	892.3	528.7
Fair Value | Baltimore Gas and Electric Company
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Fixed-rate long-term debt	$ 2,636.8	$ 2,301.8