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Marketable Securities - Classification of Marketable Securities (Detail) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Marketable Securities [Line Items] |  |
Market value | $ 3.4 | $ 2.2
Cost basis | 3.3 | 3.3
Unrealized gain (loss) | 0.1 | (1.1)
Level 1 | Common stock | Valhi |  |
Marketable Securities [Line Items] |  |
Market value | 3.3 | 2.1
Cost basis | 3.2 | 3.2
Unrealized gain (loss) | 0.1 | (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

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