Significant Accounting Policies (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 M	Dec. 31, 2010	Dec. 31, 2009
Taxes Other than Income Taxes
Gross receipts taxes and other surcharges and fees accounted for on a gross basis and included in revenues	$ 142.7	$ 122.2	$ 106.8
Earnings Per Share
Non-dilutive stock options (in shares)	4.3	5.6	5.1
Dilutive common stock equivalent shares	1.8	1.6	1.0
Stock-Based Compensation
Minimum vesting period (in years)	P1Y
Other Disclosures
Cash equivalents holding period of investments, limit at original maturity (in months)	3
Baltimore Gas and Electric Company
Taxes Other than Income Taxes
Gross receipts taxes and other surcharges and fees accounted for on a gross basis and included in revenues	$ 82.9	$ 81.9	$ 76.8