Regulatory Matters (Schedule of Regulatory Assets and Liabilities) (Details) (USD $) In Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Total Regulatory Assets	$ 178.2		$ 189.0
Total Regulatory Liabilities	133.1		124.0
Deferred Recoverable Income Taxes [Member]
Type of Recovery	B/C	[1]	B/C
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	26.6		29.9
Pension Benefits [Member]
Type of Recovery	C	[1]	C
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	76.6		81.1
Unamortized Loss on Reacquired Debt [Member]
Type of Recovery	C	[1]	C
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	13.3		14.3
Regional Transmission Organization Costs [Member]
Type of Recovery	D	[1]	D
Amortization Through	2014		2014
Total Regulatory Assets	4.4		5.5
TCRR, Transmission, Ancillary and Other PJM-related Costs [Member]
Type of Recovery	F	[1]	F
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	7.7		11.8
Deferred Storm Costs - 2008 [Member]
Type of Recovery	D	[1]	D
Total Regulatory Assets	17.7		16.9
Power Plant Emission Fees [Member]
Type of Recovery	C	[1]	C
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	7.1		6.6
CCEM Smart Grid and Advanced Metering Infrastructure Costs [Member]
Type of Recovery	D	[1]	D
Total Regulatory Assets	6.6		6.6
CCEM Energy Efficiency Program Costs [Member]
Type of Recovery	F	[1]	F
Amortization Through	Ongoing		Ongoing
Total Regulatory Assets	8.0		4.8
Fuel and Purchased Power Recovery Costs [Member]
Type of Recovery	C	[1]	C
Amortization Through	Ongoing		Ongoing
Total Regulatory Liabilities	16.0		10.0
Other Costs - Assets [Member]
Total Regulatory Assets	10.2		11.5
Estimated Costs of Removal - Regulated Property [Member]
Total Regulatory Liabilities	111.7		107.9
Postretirement Benefit [Member]
Total Regulatory Liabilities	$ 5.4		$ 6.1

[1] (a) B - Balance has an offsetting liability resulting in no impact on rate base. C - Recovery of incurred costs without a rate of return. D - Recovery not yet determined, but is probable of occurring in future rate proceedings. F - Recovery of incurred costs plus rate of return.