Fair Value Measurements and Financial Instruments - Financial Instruments not Carried at Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
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	$ 463.0	$ 471.1
Long term debt, fair value	455.1	495.1
VALHI, INC. | Contran Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	314.3	284.3
Long term debt, fair value	314.3	284.3
VALHI, INC. | Term Loan | Snake River
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount		250.0
Long term debt, fair value		250.0
Tremont | Promissory Note
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	10.9	13.1
Long term debt, fair value	10.9	13.1
BMI | Bank note payable | Meadows Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	18.0	18.8
Long term debt, fair value	18.8	19.7
LandWell | Unsecured Debt
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long term debt, carrying amount	2.3	2.5
Long term debt, fair value	2.3	2.5
Carrying Amount
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	596.1	461.7
Deferred payment obligation	9.5	9.3
Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, cash equivalents and restricted cash equivalents	596.1	461.7
Deferred payment obligation	$ 9.5	$ 9.3