Employee Incentive Plans, Compensation expense under LTI programs (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 9.7	$ 10.1	$ 13.1
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 97.52
Restricted Stock And Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 6.1	$ 5.8	$ 5.4
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount	$ 8.4
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	2 years 3 months 18 days
TSR-Based Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 134.72	$ 127.41	$ 97.88
Share-based Payment Arrangement, Expense	$ 3.6	$ 4.3	$ 7.7
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount	$ 5.4
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 10 months 24 days
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value		$ 96.40	$ 77.35
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date fair value	$ 124.10	$ 109.38	$ 86.97

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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