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Marketable Securities - Additional Information (Detail) - Valhi Inc - Common stock - $ / shares shares in Millions | Jun. 30, 2021 | Dec. 31, 2020
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Schedule Of Available For Sale Securities [Line Items] |  |
Number of shares owned | 1.2 | 1.2
Quoted marked price per share | $ 24.33 | $ 15.20

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