Common Equity (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Common Equity (Textuals) [Abstract]
Comprehensive Income, Net of Tax, Attributable to Parent	$ 92,200,000	$ 133,000,000	$ 288,000,000	$ 340,600,000
Other Comprehensive Income (Loss), Net of Tax	11,300,000	$ 0	$ 11,300,000	$ 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			195,365	233,735
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value			$ 13,200,000	$ 14,800,000
Tax Benefit from Stock Options Exercised			4,800,000	5,300,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 14,200,000		$ 14,200,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition			23 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		60,164
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value			$ 3,700,000	2,700,000
Tax Benefit from Stock Options Exercised			1,300,000	$ 1,000,000.00
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 4,600,000		$ 4,600,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition			24 months