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Long-lived Assets - Impairments, Held for Sale or Disposals - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Jun. 29, 2020	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets				$ 5.6	$ 49.7	$ 268.0
Impairment of assets related to materials & construction		$ 5.6
Goodwill			$ (65.1)	(10.5)	$ (15.4)	(65.1)
Port Allen Solar Power Facility assets | Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase consideration upon disposal		17.1
Net gain (loss) on disposal		$ 8.9
GPRM Prestress, LLC | Held for sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest sold	51.00%				51.00%
GPRM Prestress, LLC | Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase consideration upon disposal	$ 5.0
Net gain (loss) on disposal	$ (0.1)
Materials and Construction Segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill			(56.4)	(1.8)	$ (6.7)	(56.4)
Commercial property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill			(8.7)	$ (8.7)	$ (8.7)	$ (8.7)
Paving and Quarry Assets | Materials and Construction Segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets			$ 40.6
Paving and Quarry Assets | Materials and Construction Segment | Weighted average discount rate
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Weighted average discount rate (percent)					13.50%

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