Goodwill and Other Intangible Assets - Intangible Assets Deemed to Indefinite Life that are Therefore Not Amortized (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	$ 36.0	$ 28.3
Operating Permits
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	19.0
Trade names
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	2.7	2.7
Building Materials Business
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	33.5	25.8
Building Materials Business | Operating Permits
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	6.6	6.6
Building Materials Business | Use Rights Not Subject To Amortization
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	26.7	19.0
Building Materials Business | Trade names
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	0.2	0.2
Magnesia Specialties
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	2.5	2.5
Magnesia Specialties | Trade names
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	$ 2.5	$ 2.5