Share-Based Compensation - Compensation Expense for Share-Based Awards (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 30.0	$ 30.4	$ 23.5
Income tax benefit	(7.6)	(7.6)	(5.9)
Impact on net income	22.4	22.8	17.6
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	20.1	22.6	18.6
Performance Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Impact on income before income taxes	$ 9.9	$ 7.8	$ 4.9

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

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