Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2018	Apr. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2017
Income Taxes Disclosure [Line Items]
U.S. federal statutory income tax rate					21.00%		35.00%
Deferred income tax benefit from change in enacted tax rate								$ 77.1
Provision for income taxes includes non-cash deferred income tax expense					$ 7.1
Business interest income, adjusted taxable income					30.00%
Increase (decrease) in unrecognized tax benefits	$ (1.8)				$ (1.8)
Germany-Canada APA
Income Taxes Disclosure [Line Items]
Non-cash income tax benefit					1.4
ASC 740 | Discontinued Operations
Income Taxes Disclosure [Line Items]
Provision for income taxes includes non-cash deferred income tax expense					5.1
ASC 740 | Continuing Operations
Income Taxes Disclosure [Line Items]
Provision for income taxes includes non-cash deferred income tax expense					$ 2.0
Expected Future Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							$ (149.9)
Effect of Currency Exchange Rates
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance								13.7
Transition Tax | Maximum
Income Taxes Disclosure [Line Items]
Percentage of annual usage limitation on net operating loss carryforward					80.00%
Belgium | Expected Future Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							(8.0)
Germany and Belgian
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance								(186.7)
Germany and Belgian | Current Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance						$ (24.1)	(12.7)
Germany | Expected Future Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							(141.9)
Kronos Worldwide, Inc. | Germany and Belgian | Current Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance				$ (7.7)			(12.7)
European Subsidiaries | Transition Tax
Income Taxes Disclosure [Line Items]
Current income tax expense provisional pursuant to transition tax			$ 76.2
Income tax liability payable period			8 years
European Subsidiaries | Transition Tax | 2017 Tax Year [Member]
Income Taxes Disclosure [Line Items]
Current income tax expense paid		$ 6.1
European Subsidiaries | Transition Tax | 2018 Tax Year [Member]
Income Taxes Disclosure [Line Items]
Current income tax expense paid	3.1
European Subsidiaries | Transition Tax | Contran
Income Taxes Disclosure [Line Items]
Income taxes payable noncurrent	64.0				$ 64.0
Income taxes payable current	3.0				3.0
European Subsidiaries | Transition Tax | Income Tax Payable | Contran
Income Taxes Disclosure [Line Items]
Current income tax expense payable in increments over remainder of eight year period	$ 67.0				67.0
European and Canadian Subsidiaries | Undistributed Earnings Previously Considered to be Permanently Reinvested
Income Taxes Disclosure [Line Items]
Aggregate provisional non-cash deferred income tax expense			$ 5.3		2.3
Corporate Tax Purposes | Kronos Worldwide, Inc. | Germany
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards			652.0			652.0		652.0
Corporate Tax Purposes | Kronos Worldwide, Inc. | Belgium
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards			50.0			50.0		50.0
Trade Tax Purposes | Kronos Worldwide, Inc. | Germany
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards			0.5			0.5		0.5
Direct Investment in Subsidiary | Kronos Worldwide, Inc.
Income Taxes Disclosure [Line Items]
Deferred income tax liability			157.6			157.6		157.6
Deferred income taxes expense					$ 7.5		$ 39.5
Direct Investment in Subsidiary | Kronos Worldwide, Inc. | Maximum
Income Taxes Disclosure [Line Items]
Deferred income tax liability			$ 173.0			$ 173.0		$ 173.0