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Stock-Based Compensation - Allocation of Share-based Compensation Costs (Details) - USD ($) $ in Millions | 12 Months Ended
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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

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