Goodwill and Other Intangibles - Intangible Assets Deemed to Indefinite Life and Not Being Amortized (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
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