Share-Based Payments (Summary of the Restricted Stock Unit Transactions under Share-Based Payment Plans) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Incentive Plan [Member]
Weighted average remaining expense life	1 year 2 months 12 days
Change in Share Based Compensation Liability	$ 16.1
Share Based Compensation Liability Awards Amount Of Liability	8.2	$ 8.6
Cash incentive unit payments made	$ 2.3	$ 18.4	$ 20.2
Restricted Stock Units (RSUs) [Member]
Unvested RSU awards at December 31, 2019	746,725
Granted in 2020, number of shares	247,680
Vested in 2020, number of shares	(311,951)
Forfeited in 2020, number of shares	(14,780)
Unvested RSU awards at December 31, 2020, number of shares	667,674	746,725
Unvested RSU awards at December 31, 2019, weighted average grant date fair value	$ 53.48
Granted in 2020, weighted average grant date fair value	62.91	$ 63.60	$ 55.96
Vested in 2020, weighted average grant date value	44.50
Forfeited in 2020, weighted average grant date fair value	60.30
Unvested RSU awards at December 31, 2020, weighted average grant date fair value	$ 61.02	$ 53.48
Total unrecognized compensation cost related to unvested RSU awards	$ 9.2
Weighted average remaining expense life	1 year 8 months 12 days
Total intrinsic value of shares vested	$ 20.6	$ 22.0	$ 18.0
Dividend Equivalent Units [Member]
Total intrinsic value of shares vested	$ 0.7	$ 0.8	$ 0.8

Change in Share Based Compensation Liability related to the Cash Plan compared to the prior period.

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The liability recorded in connection with our Cash Incentive Plan.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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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- Definition

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 as a result of the occurrence of a terminating event.

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- Definition

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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cash paid to settle liability for award under share-based payment arrangement.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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