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Marketable Securities - Additional Information (Detail) - Valhi Inc - Common stock shares in Millions	12 Months Ended
	Dec. 31, 2020 $ / shares shares	Dec. 31, 2019 $ / shares shares
Schedule Of Available For Sale Securities [Line Items]
Number of shares owned | shares	1.2	1.2
Quoted marked price per share | $ / shares	$ 15.20	$ 22.44
Reverse stock split description	1-for-12
Reverse stock split ratio	0.083

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Balance held at close of period in number of shares.

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