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Long-lived Assets - Impairments, Held for Sale or Disposals (Tables)
12 Months Ended
Dec. 31, 2020
Discontinued Operations and Disposal Groups [Abstract]  
Long-lived Assets - Impairments of Assets Held and Used Changes to Materials & Construction fixed assets and intangible assets for the year ended December 31, 2018 including such impairments were as follows (in millions):
Intangible AssetsMaterials & ConstructionFixed AssetsMaterials & Construction
Balance, January 1, 2018$16.5 Balance, January 1, 2018$139.5 
Additions to intangible assets— Additions to fixed assets11.1 
Amortization(0.9)Depreciation(11.2)
Intangible impairment(7.0)Fixed asset impairment(33.6)
Balance, December 31, 2018$8.6 Balance, December 31, 2018$105.8