Marketable Securities - Classification of Marketable Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Marketable Securities [Line Items]
Market value	$ 4.0	$ 10.7
Cost basis	3.3	3.3
Unrealized gain	0.7	7.4
Level 1 | Common stock | Valhi
Marketable Securities [Line Items]
Market value	3.9	10.6
Cost basis	3.2	3.2
Unrealized gain	0.7	7.4
Level 1 | Common stock | NL And CompX
Marketable Securities [Line Items]
Market value	0.1	0.1
Cost basis	$ 0.1	$ 0.1