Long-Term Debt - Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Long-term debt
Total debt	$ 788.6	$ 794.3
Less current maturities	(2.4)	(4.9)
Total long-term debt	786.2	789.4
VALHI, INC. | Contran Credit Facility
Long-term debt
Total debt	270.7	313.0
Kronos Worldwide, Inc. | 3.75% Senior Secured Notes due September 15, 2025 | Kronos International, Inc
Long-term debt
Total debt	485.7	442.6
Tremont | Promissory Note
Long-term debt
Total debt		2.0
BMI | Bank loan | Western Alliance Bank
Long-term debt
Total debt	16.3	17.2
LandWell | Unsecured Debt
Long-term debt
Total debt		1.6
LandWell | Unsecured Debt | Western Alliance Bank
Long-term debt
Total debt	14.2	15.0
Other Subsidiary | Other
Long-term debt
Total debt	1.7	2.9
Subsidiary
Long-term debt
Total debt	$ 517.9	$ 481.3