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Goodwill and Other Intangibles - Intangible Assets Deemed to Indefinite Life and Not Being Amortized (Detail) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | $ 36.0 | $ 36.0
Building Materials Business |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 33.5 | 33.5
Magnesia Specialties |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 2.5 | 2.5
Operating Permits |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 6.6 | 6.6
Operating Permits | Building Materials Business |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 6.6 | 6.6
Use Rights Not Subject To Amortization |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 26.7 | 26.7
Use Rights Not Subject To Amortization | Building Materials Business |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 26.7 | 26.7
Trade names |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 2.7 | 2.7
Trade names | Building Materials Business |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | 0.2 | 0.2
Trade names | Magnesia Specialties |  |
Indefinite Lived Intangible Assets By Major Class [Line Items] |  |
Indefinite life intangible assets | $ 2.5 | $ 2.5

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 + Details
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