Share-Based Compensation and Stock Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Share-Based Compensation
Upon vesting, each restricted stock unit and performance share replaced with common stock	1.00%
Common stock repurchased (in shares)	177,580	281,000
Common stock repurchased, value	$ 10.0	$ 10.0
Payment of income tax obligation on employee equity awards	$ 13.5	3.8
Employee stock option
Share-Based Compensation
Options granted (in shares)	0
Restricted stock units
Share-Based Compensation
Granted (in dollars per share)	$ 53.40
Restricted stock units \| Long Term Incentive Plan 2010
Share-Based Compensation
Granted (in shares)	114,856
Vesting period	3 years
Performance share units \| Long Term Incentive Plan 2010
Share-Based Compensation
Granted (in shares)	89,982
Vesting period	3 years
Performance share units \| Minimum \| Long Term Incentive Plan 2010
Share-Based Compensation
Percentage number of performance shares awarded	0.00%
Performance share units \| Maximum \| Long Term Incentive Plan 2010
Share-Based Compensation
Percentage number of performance shares awarded	200.00%
Director
Share-Based Compensation
Granted (in shares)	15,165
Granted (in dollars per share)	$ 53.40
Accounting Standards Update 2017-09
Share-Based Compensation
Stock based compensation expense	$ 7.6	$ 5.8

Upon vesting, each restricted stock unit and performance share replaced with common stock.

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The number of performance shares awarded represented as a percentage of the original granted amount based on performance.

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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.

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Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Gross number of share options (or share units) granted during the period.

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Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

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