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Inventories (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Inventory [Line Items]    
Inventories $ 18.4 $ 20.7
Asphalt    
Inventory [Line Items]    
Inventories 4.2 8.0
Processed rock and sand    
Inventory [Line Items]    
Inventories 7.9 6.6
Work in progress    
Inventory [Line Items]    
Inventories 3.2 2.9
Retail merchandise    
Inventory [Line Items]    
Inventories 2.1 2.0
Parts, materials and supplies inventories    
Inventory [Line Items]    
Inventories $ 1.0 $ 1.2