Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Long-term debt
Total debt	$ 765.5	$ 788.6
Less current maturities	(2.4)	(2.4)
Total long-term debt	763.1	786.2
VALHI, INC. | Contran Credit Facility
Long-term debt
Total debt	268.4	270.7
Kronos Worldwide, Inc. | 3.75% Senior Secured Notes due September 15, 2025 | Kronos International, Inc
Long-term debt
Total debt	465.0	485.7
BMI | Bank loan | Western Alliance Bank
Long-term debt
Total debt	16.3	16.3
LandWell | Unsecured Debt | Western Alliance Bank
Long-term debt
Total debt	14.2	14.2
Other Subsidiary | Other
Long-term debt
Total debt	1.6	1.7
Subsidiary
Long-term debt
Total debt	$ 497.1	$ 517.9