Fair Value Measurements and Financial Instruments - Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Kronos Worldwide, Inc. | 3.75% Senior Secured Notes due September 15, 2025 | Kronos International, Inc
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	$ 465.0	$ 485.7
Long term debt, fair value	478.1	499.9
VALHI, INC. | Contran Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	268.4	270.7
Long term debt, fair value	268.4	270.7
BMI | Bank note payable | Meadows Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	16.3	16.3
Long term debt, fair value	16.9	16.9
LandWell | Bank note payable
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	14.2	14.2
Long term debt, fair value	14.2	14.2
Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	600.9	570.3
Deferred payment obligation		1.3
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	$ 600.9	570.3
Deferred payment obligation		$ 1.3