Income Taxes - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended			12 Months Ended
	Mar. 27, 2020	Mar. 26, 2020	Mar. 31, 2021	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax [Line Items]
Deferred tax assets valuation allowance						$ 7.4	$ 4.4
Transition tax					$ 74.5
Income tax liability payable period						8 years
Noncurrent payable to affiliate						$ 44.7	50.6
Current payable to affiliate						18.2	27.9
Current cash tax benefit						(21.6)	(14.9)	$ (21.9)
Non-cash deferred income tax benefit						(11.0)	1.0	$ (5.8)
Business interest income, adjusted taxable income		30.00%
CARES act of 2020, Business interest income, adjusted taxable income	50.00%
Cash tax benefit due to reversal of valuation allowance			$ 0.5
Increase decrease in valuation allowance							$ 2.8
Estimated unrecognized tax benefits decrease amount for next twelve months						1.1
Unrecognized tax benefits that would impact effective tax rate						3.8
Valhi | Income Tax Payable
Income Tax [Line Items]
Payable to affiliate						50.6
Noncurrent payable to affiliate						44.7
Current payable to affiliate						5.9
Corporate Purposes
Income Tax [Line Items]
Deferred tax assets valuation allowance						0.0
Germany
Income Tax [Line Items]
Income tax benefit				$ 3.0
Current cash tax benefit				1.5
Non-cash deferred income tax benefit				1.5
Non-cash deferred income tax expense				$ 5.5
Germany | Corporate Purposes
Income Tax [Line Items]
Net operating loss carryforwards						451.0
Belgium | Corporate Purposes
Income Tax [Line Items]
Net operating loss carryforwards						$ 19.0