Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 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	$ 5.9
Business interest income, adjusted taxable income	30.00%
Increase (decrease) in unrecognized tax benefits	$ (1.8)
Transition Tax | Maximum
Income Taxes Disclosure [Line Items]
Percentage of annual usage limitation on net operating loss carryforward	80.00%
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			$ (5.0)
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 | Income Tax Payable | Contran
Income Taxes Disclosure [Line Items]
Current income tax expense payable in increments over remainder of eight year period		$ 70.1		70.1
European Subsidiaries | Transition Tax | Income Tax Receivable | Contran
Income Taxes Disclosure [Line Items]
Current income tax expense payable in next fiscal year		6.1		6.1
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	$ 0.8	5.3
Corporate Tax Purposes | Kronos Worldwide, Inc. | Germany
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards		652.0		652.0
Corporate Tax Purposes | Kronos Worldwide, Inc. | Belgium
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards		50.0		50.0
Trade Tax Purposes | Kronos Worldwide, Inc. | Germany
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards		0.5		0.5
Direct Investment in Subsidiary | Kronos Worldwide, Inc.
Income Taxes Disclosure [Line Items]
Deferred income tax liability		157.6		157.6
Deferred income taxes expense	$ 3.6		$ 10.0
Direct Investment in Subsidiary | Kronos Worldwide, Inc. | Maximum
Income Taxes Disclosure [Line Items]
Deferred income tax liability		$ 173.0		$ 173.0