Stock-Based Compensation - Stock Award Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-Based Compensation [Abstract]
Requisite service period			3 years
Stock-based compensation expense	$ 4.1	$ 3.6	$ 12.1	$ 11.0
Stock-based compensation expense for retirement savings plan contributions	0.7	0.6	2.1	1.9
Associated current and future income tax benefits recognized	0.8	$ 1.1	2.4	$ 3.4
Total unrecognized compensation cost related to non-vested share-based awards	$ 17.1		$ 17.1
Unrecognized compensation cost, weighted-average period for recognition			2 years
2012 Long Term Incentive Plan
Stock-Based Compensation [Abstract]
Maximum number of shares authorized under plan (in shares)	7.0		7.0
Number of shares available for issuance under the Incentive Plan	2.6		2.6

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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-Section 50
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Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Amount of cost (reversal of cost) for pension and other postretirement benefits.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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