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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net periodic postretirement benefit cost | $ 5.2 | $ 7.0 | $ 6.6
Total other employee benefit plans | 19.9 | 22.4 | 21.0
Canadian pension plan | Canadian pension plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net periodic postretirement benefit cost | 0.4 | 0.4 | 0.4
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost | 8.3 | 8.8 | 8.8
Deferred compensation arrangement with individual, compensation expense | 1.5 | 1.6 | 1.0
Multiemployer plan, contributions by employer | 4.9 | 5.0 | 4.7
Postretirement Health Coverage | Salaried VEBA |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net periodic postretirement benefit cost | $ 4.8 | $ 6.6 | $ 6.1

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