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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Other Postretirement Benefits Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Defined contribution plan, cost | $ 3.4 | $ 3.9
Deferred compensation arrangement with individual, compensation expense |  | (0.4)
Multiemployer plan, contributions by employer | 1.1 | 1.3
Total other employee benefit plans | 5.1 | 6.0
Postretirement Health Coverage | Salaried VEBA |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Total net periodic postretirement benefit cost relating to Salaried VEBA | $ 0.6 | $ 1.2

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