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