Fair Value of Financial Instruments (Details) (USD $) In Millions	Sep. 30, 2011
Baltimore Gas and Electric Company | Carrying Amount
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Fixed-rate long-term debt	$ 2,113.9
Carrying Amount
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Investments and other assets	233.5
Fixed-rate long-term debt	3,843.0
Variable-rate long-term debt	907.3
Baltimore Gas and Electric Company | Fair Value
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Fixed-rate long-term debt	2,365.3
Fair Value
The carrying amounts and fair values of financial instruments included in our Consolidated Balance Sheets
Investments and other assets	233.8
Fixed-rate long-term debt	4,301.2
Variable-rate long-term debt	$ 907.3