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Marketable Securities - Classification of Marketable Securities (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Marketable Securities [Line Items]    
Market value $ 2.0 $ 3.3
Cost basis 3.3 3.3
Unrealized loss (1.3)  
Level 1 | Common stock | Valhi    
Marketable Securities [Line Items]    
Market value 1.9 3.2
Cost basis 3.2 3.2
Unrealized loss (1.3)  
Level 1 | Common stock | NL And CompX    
Marketable Securities [Line Items]    
Market value 0.1 0.1
Cost basis $ 0.1 $ 0.1