Noncontrolling Interest in Subsidiaries - Noncontrolling Interest in Net Assets of Subsidiaries (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	$ 313.2	$ 324.4
Kronos Worldwide, Inc.
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	217.2	212.3
NL Industries
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	72.3	67.1
CompX International
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	22.5	23.5
BMI
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	6.8	14.7
LandWell
Noncontrolling interest in net assets:
Noncontrolling interest in subsidiaries	$ (5.6)	$ 6.8