Regulatory Assets & Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets [Abstract]
Regulatory assets	$ 226.0	$ 189.4
Regulatory Assets [Abstract]
Weighted average recovery period of regulatory assets currently being recovered (in years)	17
Regulatory Liabilities [Abstract]
Regulatory liabilities	345.2	333.5
Asset Retirement Obligations and Other [Member]
Regulatory Liabilities [Abstract]
Regulatory liabilities	320.9	307.5
Future Amounts Recoverable From Ratepayers [Member]
Regulatory Assets [Abstract]
Regulatory assets	129.6	113.8
Amounts Deferred for Future Recovery [Member]
Regulatory Assets [Abstract]
Regulatory assets	24.1	2.8
Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	72.3	72.8
Benefit Obligations [Member] | Future Amounts Recoverable From Ratepayers [Member]
Regulatory Assets [Abstract]
Regulatory assets	126.0	92.5
Deferred Income Tax Charges [Member] | Future Amounts Recoverable From Ratepayers [Member]
Regulatory Assets [Abstract]
Regulatory assets	1.3	19.2
Asset Retirement Obligations and Other [Member] | Future Amounts Recoverable From Ratepayers [Member]
Regulatory Assets [Abstract]
Regulatory assets	2.3	2.1
Cost Recovery Riders Other [Member] | Amounts Deferred for Future Recovery [Member]
Regulatory Assets [Abstract]
Regulatory assets	6.4	2.8
Unamortized Debt Issue Costs and Hedging Proceeds [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	34.3	35.7
Demand Side Management Programs [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	6.3	9.5
Indiana Authorized Trackers [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	24.3	17.3
Ohio Authorized Trackers [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	1.0	2.0
Premiums Paid to Reaquire Debt [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	3.3	3.8
Other Base Rate Recoveries [Member] | Amounts Currently Recovered in Customer Rates [Member]
Regulatory Assets [Abstract]
Regulatory assets	3.1	4.5
Deferred Coal Costs [Member] | Amounts Deferred for Future Recovery [Member]
Regulatory Assets [Abstract]
Regulatory assets	$ 17.7	$ 0