Income Taxes - Additional Information (Detail) CAD in Millions, $ in Millions		3 Months Ended						6 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2017 CAD
Income Taxes Disclosure [Line Items]
U.S. federal statutory income tax rate		35.00%		35.00%					35.00%	35.00%
Estimated reversal of deferred income tax asset valuation allowance		$ 11.7							$ 11.7
Income tax examination, description									Tax authorities are examining certain of our U.S. and non-U.S. tax returns and have or may propose tax deficiencies, including penalties and interest. Because of the inherent uncertainties involved in settlement initiatives and court and tax proceedings, we cannot guarantee that these matters will be resolved in our favor, and therefore our potential exposure, if any, is also uncertain. As a result of ongoing audits in certain jurisdictions, in 2008 Kronos filed Advance Pricing Agreement Requests with the tax authorities in the U.S., Canada and Germany. These requests have been under review with the respective tax authorities since 2008 and prior to 2016, it was uncertain whether an agreement would be reached between the tax authorities and whether we would agree to execute and finalize such agreements.
Income taxes	$ 5.1	22.9							$ 22.9		$ 5.1
Increase (decrease) in unrecognized tax benefits		(8.0)							$ (8.0)
US-Canada APA
Income Taxes Disclosure [Line Items]
Current U.S. income tax benefit				$ (5.6)
Canada - Germany APA
Income Taxes Disclosure [Line Items]
U.S.-Canada Advance Pricing Agreement, description									During the third quarter of 2017, our 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.
Non-cash income tax benefit related to reversal of reserve for uncertain tax positions		8.1
Earliest Tax Year | Canada - Germany APA
Income Taxes Disclosure [Line Items]
Effective tax year									2005
Latest Tax Year | Canada - Germany APA
Income Taxes Disclosure [Line Items]
Effective tax year									2017
Germany | Canada - Germany APA
Income Taxes Disclosure [Line Items]
Non-cash income tax benefit related to increase in German NOLs		2.6
Cash tax refund		0.6
Germany and Belgium
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance									$ (170.4)
Kronos Worldwide, Inc. | Earliest Tax Year | US-Canada APA
Income Taxes Disclosure [Line Items]
Effective tax year				2005
Kronos Worldwide, Inc. | Latest Tax Year | US-Canada APA
Income Taxes Disclosure [Line Items]
Effective tax year				2015
Kronos Worldwide, Inc. | Direct investment in subsidiary excess carrying amount
Income Taxes Disclosure [Line Items]
Deferred income taxes expense (benefit)					$ (29.3)	$ (29.3)	$ (29.3)		49.3	$ 9.1	6.5
Kronos Worldwide, Inc. | Germany | Corporate Tax Purposes
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	638.0										638.0
Kronos Worldwide, Inc. | Germany | Trade Tax Purposes
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	71.0										71.0
Kronos Worldwide, Inc. | Belgium | Corporate Tax Purposes
Income Taxes Disclosure [Line Items]
Net operating loss carryforwards	93.0										93.0
Kronos Canadian Subsidiary | US-Canada APA
Income Taxes Disclosure [Line Items]
Income taxes		2.3							2.3			CAD 3
Current Periods Net Operating Loss Utilization | Germany and Belgium
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance		(7.8)						$ (12.7)
Current Periods Net Operating Loss Utilization | Kronos Worldwide, Inc. | Germany and Belgium
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance		$ (7.8)									$ (2.2)
Deferred income taxes expense (benefit)								$ (12.7)
Expected Future Periods Net Operating Loss Utilization
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance			$ (149.9)
Expected Future Periods Net Operating Loss Utilization | Germany
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance			(141.9)
Expected Future Periods Net Operating Loss Utilization | Belgium
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance			(8.0)
Expected Future Periods Net Operating Loss Utilization | Kronos Worldwide, Inc.
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance	(173.0)		$ (149.9)
Expected Future Periods Net Operating Loss Utilization | Kronos Worldwide, Inc. | Germany
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance	(153.0)
Expected Future Periods Net Operating Loss Utilization | Kronos Worldwide, Inc. | Belgium
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance	$ (20.0)
Effect of Currency Exchange Rates
Income Taxes Disclosure [Line Items]
Deferred income tax asset valuation allowance									$ 9.0