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Business and geographic segments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Segment Reporting Information [Line Items]
Loss on deconsolidation of BWC		$ 0.0	$ (2.0)	$ 0.0	$ (2.0)
BMI
Segment Reporting Information [Line Items]
Loss on deconsolidation of BWC						$ 19.7
Sale of business	$ 2.0
Kronos Worldwide, Inc. \| Valhi Incorporation
Segment Reporting Information [Line Items]
Direct ownership percentage by parent		50.00%		50.00%
Kronos Worldwide, Inc. \| NL
Segment Reporting Information [Line Items]
Indirect controlling interest in subsidiary		31.00%		31.00%
NL \| Valhi Incorporation
Segment Reporting Information [Line Items]
Direct ownership percentage by parent		83.00%		83.00%
LandWell \| Valhi Incorporation
Segment Reporting Information [Line Items]
Direct ownership percentage by parent		27.00%		27.00%
LandWell \| BMI
Segment Reporting Information [Line Items]
Indirect controlling interest in subsidiary		50.00%		50.00%
BMI
Segment Reporting Information [Line Items]
Loss on deconsolidation of BWC			2.0
Bad debt expense recognized			$ 1.3
BMI \| Valhi Incorporation
Segment Reporting Information [Line Items]
Direct ownership percentage by parent		63.00%		63.00%