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Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | $ 77.1 | $ 70.6 |
Salaried VEBA |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 60.5 | 57.6 | $ 53.2
Postretirement Health Coverage | Salaried VEBA | Fixed Income Funds | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 22.7 | 21.9 |
Postretirement Health Coverage | Salaried VEBA | Equity Funds | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 34.7 | 24.8 |
Canadian pension plan | Diversified Investment Funds in Registered Investment Companies | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 8.7 | 7.8 |
Level 1 | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 1.4 | 8.0 |
Level 1 | Salaried VEBA | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 8.0 |
Level 1 | Postretirement Health Coverage | Salaried VEBA | Cash and Cash Equivalents | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 1.4 | 0.9 |
Level 1 | Postretirement Health Coverage | Salaried VEBA | Diversified Investment Funds in Registered Investment Companies | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 7.1 |
Level 2 | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 9.6 | 8.1 |
Deferred Compensation Plan Assets | 9.6 | 8.1 |
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | 11.0 | 16.1 |
Deferred Compensation Plan Assets | 9.6 | 8.1 |
Fair Value, Inputs, Level 1, 2 and 3 | Salaried VEBA | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 8.0 |
Fair Value, Inputs, Level 1, 2 and 3 | Postretirement Health Coverage | Salaried VEBA | Cash and Cash Equivalents | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount | $ 1.4 | 0.9 |
Fair Value, Inputs, Level 1, 2 and 3 | Postretirement Health Coverage | Salaried VEBA | Diversified Investment Funds in Registered Investment Companies | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | $ 7.1 |

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