Employee Incentive Plans, Compensation expense under LTI programs (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense	$ 10.1	$ 13.1	$ 11.7
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 96.40
Restricted Stock And Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense	$ 5.8	$ 5.4	$ 4.7
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 8.7
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 6 months
TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 127.41	$ 97.88	$ 93.02
Allocated Share-based Compensation Expense	$ 4.3	$ 7.7	$ 7.0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 6.0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	1 year 7 months 6 days
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value		$ 77.35	$ 76.82
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 109.38	$ 86.97	$ 88.00

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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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 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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