Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended					6 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 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		$ (59.7)							$ (59.7)		$ 77.1
Adjustment of deferred income tax expense from change in enacted tax rate											59.7
Current income tax expense provisional pursuant to transition tax		2.1							2.1
Income tax expense (benefit)		(91.4)			$ 14.7				(33.2)	$ (67.5)
Provision for income taxes includes non-cash deferred income tax expense									$ 3.0
Business interest income, adjusted taxable income									30.00%
Effective state income tax rate		$ (3.4)							$ (3.4)
Effective income tax rate		38.00%							38.90%
Germany-Canada APA
Income Taxes Disclosure [Line Items]
U.S.-Canada Advance Pricing Agreement, description									During the third quarter of 2017, Kronos’ Canadian subsidiary executed and finalized an Advance Pricing Agreement with the Competent Authority for Canada (the “Canada-Germany APA”) effective for tax years 2005 - 2017. Pursuant to the terms of the Canada-Germany APA, the Canadian and German tax authorities agreed to certain prior year changes to taxable income of our Canadian and German subsidiaries.
Income tax expense (benefit) related to increase (decrease) in reserve for uncertain tax positions					8.1
Increase (decrease) in unrecognized tax benefits		$ (1.8)							$ (1.8)
Non-cash income tax benefit		1.4
Earliest Tax Year | Germany-Canada APA
Income Taxes Disclosure [Line Items]
Effective tax year									2005
Latest Tax Year | Germany-Canada APA
Income Taxes Disclosure [Line Items]
Effective tax year									2017
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					(7.8)		$ (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
Income tax expense (benefit)		2.1
Income tax liability payable period									8 years
Aggregate transition tax installments payments made		10.6							$ 10.6
Payable to affiliate		63.5							63.5
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		4.5	$ 4.5
European Subsidiaries | Transition Tax | Contran
Income Taxes Disclosure [Line Items]
Income taxes payable noncurrent		57.8							57.8
Income taxes payable current		5.7							5.7
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		63.5							63.5
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					3.2
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		$ 113.0							$ 19.2	$ 49.3
Direct Investment in Subsidiary | Kronos Worldwide, Inc. | Maximum
Income Taxes Disclosure [Line Items]
Deferred income tax liability				$ 155.4			$ 155.4				$ 155.4
Canada Revenue Agency and German Federal Central Tax Office | Germany | Germany-Canada APA
Income Taxes Disclosure [Line Items]
Non-cash income tax benefit related to increase in German NOLs					2.6
Cash tax refund					$ 0.6