Stock-Based Compensation - Allocation of Share-based Compensation Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 34.0	$ 24.4	$ 15.1
Stock option and appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	10.2	9.1	8.6
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	16.6	9.5	2.5
Cash-Settled PSU Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	0.4	0.1	0.0
Performance-vesting Shares or Units Dividend Equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	0.6	0.2	0.1
Employee Stock Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	1.1	0.9	0.9
Deferred Compensation, Share-based Payments
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 5.1	$ 4.6	$ 3.0