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Long-lived Assets - Impairments, Held for Sale or Disposals - Schedule of Intangible Assets and Fixed Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward]
Balance, January 1, 2018	$ 74.9
Amortization	(14.5)	$ (12.5)	$ (8.7)
Balance, December 31, 2018	61.9	74.9
Movement in Property, Plant and Equipment [Roll Forward]
Additions to fixed assets	25.1	255.1	296.1
Depreciation	(38.8)	(35.6)	(32.5)
Materials and Construction Segment
Finite-lived Intangible Assets [Roll Forward]
Balance, January 1, 2018	8.6	16.5
Additions to intangible assets		0.0
Amortization		(0.9)
Intangible impairment		(7.0)
Balance, December 31, 2018		8.6	16.5
Movement in Property, Plant and Equipment [Roll Forward]
Balance, January 1, 2018	$ 105.8	139.5
Additions to fixed assets		11.1
Depreciation		(11.2)
Fixed asset impairment		(33.6)
Balance, December 31, 2018		$ 105.8	$ 139.5

X

- Definition

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

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The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

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Long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets.

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Amount of acquisition of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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