Employee Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Defined Benefit Plan, Plan Assets, Amount	$ 79.5	$ 77.1
Salaried VEBA
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Defined Benefit Plan, Plan Assets, Amount	59.2	60.5	$ 57.6
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
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
Pension Plan
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Defined Benefit Plan, Plan Assets, Amount	9.8	8.7	$ 7.8
Pension Plan | 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	0.1
Pension Plan | 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	0.1
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	9.6	8.7
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	69.7	1.4
Deferred Compensation Plan Assets	10.5
Level 1 | 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	38.6
Level 1 | 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	20.6
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
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
Deferred Compensation Plan Assets		9.6
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	69.7	11.0
Deferred Compensation Plan Assets	10.5	9.6
Fair Value, Inputs, Level 1, 2 and 3 | 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	38.6
Fair Value, Inputs, Level 1, 2 and 3 | 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	$ 20.6
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