Common Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	$ 8.3	$ 12.5	$ 14.9	$ 27.7
Related Tax Benefit	3.4	5.0	6.0	11.1
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			9.0	20.2
Comprehensive Income, Net of Tax, Attributable to Parent			410.4	330.9
Stock options [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	0.6	1.9	1.9	5.7
Performance units [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	7.3	10.3	11.7	20.9
Common Equity (Textuals) [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period			435,690	555,830
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Intrinsic Value			$ 12.6	$ 9.8
Tax Benefit from Stock Options Exercised			4.3	3.4
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	19.2		19.2
Employee Service Share Based Compensation Unrecognized Compensation Costs On Performance Units Awards Weighted Average Period Of Recognition			20
Restricted stock [Member]
Recorded pre-tax share-based compensation expense and related tax benefit for share-based awards made to employees and directors
Share-based compensation expense	0.4	0.3	1.3	1.1
Common Equity (Textuals) [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	0		74,850
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Intrinsic Value	$ 0	$ 0.1	$ 2.3	$ 1.1
Tax Benefit from Stock Options Exercised	0	0.1	0.7	0.3
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 2.9		$ 2.9

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the reporting entity. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, but excludes any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

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-Name Accounting Research Bulletin (ARB)

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The intrinsic value of equity-based payment equity instruments, excluding stock (or unit) options, that vested during the reporting period as calculated by applying the disclosed pricing methodology.

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Reductions in the entity's income taxes that arise when compensation cost (from non-qualified stock options) recognized on the entity's tax return exceeds compensation cost from non-qualified stock options recognized on the income statement. This element increases net cash provided by operating activities.

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Common Equity.

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