Goodwill and Other Intangible Assets - Intangible Assets Deemed to Indefinite Life that are Therefore Not Amortized (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	$ 28.3	$ 29.7
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	19.0	20.3
Trade names
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	2.7	2.8
Building Materials Business
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	25.8	27.2
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	19.0	20.3
Building Materials Business | Trade names
Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite life intangible assets	0.2	0.3
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