Financial Instruments - Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
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	$ 485.7	$ 442.6
Long term debt, fair value	499.9	457.0
VALHI, INC. | Contran Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	270.7	313.0
Long term debt, fair value	270.7	313.0
Tremont | Promissory Note
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount		2.0
Long term debt, fair value		2.0
BMI | Bank note payable | Meadows Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	16.3	17.2
Long term debt, fair value	16.9	17.9
LandWell | Bank note payable
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	14.2	15.0
Long term debt, fair value	14.2	15.0
LandWell | Unsecured Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount		1.6
Long term debt, fair value		1.6
Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	570.3	583.8
Deferred payment obligation	1.3	9.9
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	570.3	583.8
Deferred payment obligation	$ 1.3	$ 9.9