Common Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Common Equity (Textuals) [Abstract]
Minimum percentage award based on performance unit			0.00
Maximum percentage award based on performance unit			1.75
Tax Benefit from Stock Options Exercised			$ 0	$ 9.0
Other Comprehensive Income (Loss), Net of Tax	0	0	0	0
Performance units [Member]
Common Equity (Textuals) [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period			346,570	435,690
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Intrinsic Value			26.7	12.6
Tax Benefit from Stock Options Exercised			9.7	4.3
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	18.4		18.4
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition			19 months
Restricted stock [Member]
Common Equity (Textuals) [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	0		94,959
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Intrinsic Value	0	0	3.3	2.3
Tax Benefit from Stock Options Exercised	0	0	0	0.7
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 3.3		$ 3.3
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition			22 months