Marketable Securities - Classification of Marketable Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Marketable Securities [Line Items]
Market value	$ 3.0	$ 2.2
Cost basis	3.3	3.3
Unrealized loss	(0.3)	(1.1)
Level 1 | Common stock | Valhi
Marketable Securities [Line Items]
Market value	2.9	2.1
Cost basis	3.2	3.2
Unrealized loss	(0.3)	(1.1)
Level 1 | Common stock | NL And CompX
Marketable Securities [Line Items]
Market value	0.1	0.1
Cost basis	$ 0.1	$ 0.1