Valhi Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Millions			12 Months Ended
	Aug. 10, 2020 $ / shares shares	May 28, 2020	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) $ / shares shares	Jun. 30, 2020 shares	Dec. 31, 2012 shares
Schedule Of Stockholders Equity [Line Items]
Reverse stock split, description		Reverse stock split. On May 28, 2020 following stockholder approval at our annual meeting, our board of directors approved a reverse stock split of our common stock at a ratio of 1-for-12, which was effective on June 1, 2020.
Reverse stock split, conversion ratio		12
Reclassification from common stock to additional paid-in capital | $			$ 3.3
Common stock, par value | $ / shares			$ 0.01	$ 0.01
Treasury stock, shares			1,100,000	1,100,000	1,100,000
Percentage of shares held by majority-owned subsidiary that has to be considered for voting purpose			100.00%
Preferred stock, par value | $ / shares			$ 0.01	$ 0.01
Director Plan
Schedule Of Stockholders Equity [Line Items]
Number of shares authorized								200,000
Shares awarded under new plan			50,000	50,000	14,500
Common stock available for future grant			24,000
Contran
Schedule Of Stockholders Equity [Line Items]
Preferred stock, par value | $ / shares	$ 0.01
Non-cumulative dividend on preferred stock	6.00%
Preferred stock returned to contribution agreement authorized designated	5,000
Series A Preferred Stock
Schedule Of Stockholders Equity [Line Items]
Preferred stock, shares outstanding						5,000
Preferred stock liquidation preference, per share | $ / shares						$ 133,466.75
Preferred stock, aggregate liquidation preference | $						$ 667.3
Preferred stock, par value | $ / shares						$ 0.01
Non-cumulative dividend on preferred stock						6.00%
NL
Schedule Of Stockholders Equity [Line Items]
Treasury stock, shares			1,200,000	1,200,000
Unrealized losses of shares attributable to noncontrolling interest | $			$ 1.7	$ 0.2	$ 12.2
NL | Director Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for future grant			79,900
Kronos Worldwide, Inc.
Schedule Of Stockholders Equity [Line Items]
Shares available for purchase			1,560,000
Treasury stock, shares			100,000	100,000
Unrealized losses of shares attributable to noncontrolling interest | $			$ 1.7	$ 0.2	$ 12.2
Kronos Worldwide, Inc. | Director Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for future grant			127,400
CompX | Director Plan
Schedule Of Stockholders Equity [Line Items]
Common stock available for future grant			149,050
Common stock
Schedule Of Stockholders Equity [Line Items]
Shares available for purchase			334,000				833,333