Inventories (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Inventory [Line Items]
Inventories	$ 20.2	$ 20.7
Asphalt
Inventory [Line Items]
Inventories	7.2	8.0
Processed rock and sand
Inventory [Line Items]
Inventories	6.6	6.6
Work in progress
Inventory [Line Items]
Inventories	3.4	2.9
Retail merchandise
Inventory [Line Items]
Inventories	2.0	2.0
Parts, materials and supplies inventories
Inventory [Line Items]
Inventories	$ 1.0	$ 1.2