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Marketable Securities - Classification of Marketable Securities (Detail) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Marketable Securities [Line Items]    
Market value $ 5.2 $ 3.4
Cost basis 3.3 3.3
Unrealized gain 1.9 0.1
Level 1 | Common stock | Valhi    
Marketable Securities [Line Items]    
Market value 5.1 3.3
Cost basis 3.2 3.2
Unrealized gain 1.9 0.1
Level 1 | Common stock | NL And CompX    
Marketable Securities [Line Items]    
Market value 0.1 0.1
Cost basis $ 0.1 $ 0.1