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Income Taxes - Tax Effects of Significant Temporary Differences Creating Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Deferred Tax Assets, Gross [Abstract]    
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits $ 3.4 $ 3.8
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits 19.2 17.1
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation 5.2 4.4
Deferred Tax Assets, Inventory 0.6 7.6
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions 0.2 3.2
Deferred Tax Assets, Tax Credit Carryforwards 16.2 10.3
Deferred Tax Assets, Operating Loss Carryforwards 3.9 8.8
Deferred tax assets, gross 68.5 89.7
Deferred Tax Assets, Valuation Allowance 5.4 5.3
Deferred Tax Assets, Net of Valuation Allowance 63.1 84.4
Deferred tax liabilities:    
Plant and equipment (199.7) (179.2)
Deferred Tax Liabilities, Leasing Arrangements (16.4) (18.9)
Intangible assets (3.0) (3.8)
Deferred Tax Liabilities, Gross 219.1 201.9
Deferred Tax Liabilities, Net 156.0 117.5
Interest Expense [Member]    
Deferred Tax Assets, Gross [Abstract]    
Deferred Tax Assets, Tax Deferred Expense, Other 0.0 12.4
Lease Agreements [Member]    
Deferred Tax Assets, Gross [Abstract]    
Deferred Tax Assets, Other 18.0 20.5
Other Expense [Member]    
Deferred Tax Assets, Gross [Abstract]    
Deferred Tax Assets, Other $ 1.8 $ 1.6