Stock Based Compensation - Restricted Stock Units (Details) - Long-Term Equity Incentive Plan - Restricted Stock Units - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 7.4	$ 8.7	$ 14.4	$ 13.8
Income tax benefit	(1.4)	(1.5)	(2.7)	(2.3)
Compensation expense, net of tax	$ 6.0	$ 7.2	$ 11.7	$ 11.5
RSUs granted (in shares)	31,554	32,330	225,396	421,829
Weighted average grant date fair value per unit (in dollars per share)	$ 96.18	$ 89.56	$ 97.56	$ 90.64
Total fair value of vested RSUs	$ 5.0	$ 2.6	$ 26.8	$ 18.8
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized [Abstract]
Unrecognized compensation cost	$ 31.2		$ 31.2
Unrecognized compensation cost expected to be recognized over a weighted-average period (in years)			1 year 3 months 18 days

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 amount of expense, net of income tax, 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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Unrecognized cost of unvested share-based compensation awards.

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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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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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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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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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