Share-Based Compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of changes in restricted stock and related information
Compensation expense	$ 19.5	$ 16.7	$ 14.4
Restricted stock
Share-based compensation
Percentage of restricted stock vesting two years from the grant date	25.00%
Percentage of restricted stock vesting on each of the next three consecutive grant anniversary dates	25.00%
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	717,000
Granted (in shares)	336,000
Vested (in shares)	(157,245)	(115,764)	(103,418)
Forfeited (in shares)	(21,000)
Unvested at the end of the period (in shares)	875,000	717,000
Weighted Average Grant Date Fair Value, beginning of the period (in dollars per share)	$ 45.04
Weighted Average Grant Date Fair Value Granted (in dollars per share)	$ 60.56	$ 50.75	$ 27.81
Weighted Average Grant Date Fair Value Vested (in dollars per share)	$ 49.26
Weighted Average Grant Date Fair Value Forfeited (in dollars per share)	$ 50.31
Weighted Average Grant Date Fair Value, end of the period (in dollars per share)	$ 50.12	$ 45.04
Total fair value of awards vested during the period	7.7	7.7	7.2
Compensation expense	10.1	8.4	7.3
Unrecognized compensation cost	$ 25.9
Period for recognition of unrecognized compensation cost (in years)	3.5

Represents the percent of the total restricted stock that in general vests two years from the date of grant.

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Represents the remaining percent of the total restricted stock that vests on each of the next three consecutive grant anniversary dates.

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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.

+ References

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan.

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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 weighted average fair value at grant date for nonvested equity-based awards issued 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 number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The total fair value of equity-based awards for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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