Common Equity - Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Cash received from option exercises			$ 26.4	$ 31.7
Stock options [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	6,750,930		6,770,194
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period	0		516,475	899,500
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(599,752)		(1,135,491)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	6,151,178		6,151,178
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 32.32		$ 29.99
Share Based Compensation Arrangement By Share Based Payment Award Options Grant In Period Weighted Average Exercise Price	0.00		52.90
Share Based Compensation Arrangement By Share Based Payment Awards Options Exercises In Period Weighted Average Exercise Price	23.72		23.25
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 33.16		$ 33.16
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range Outstanding Options Weighted Average Remaining Contractual Period			5 years 9 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 117.2		$ 117.2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	3,419,728		3,419,728
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 26.49		$ 26.49
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Remaining Contractual Term			4 years
Share Based Compensation Arrangement by Share Based Payments Award, Options Exercisable, Intrinsic Value	$ 88.0		$ 88.0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	15.8	$ 12.7	31.2	$ 30.2
Cash received from option exercises			26.4	31.7
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options			12.5	$ 12.1
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 2.3		$ 2.3
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition			17 months
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			516,475
Exercise price of options that were anti-dilutive	$ 52.90		$ 52.90