Income Taxes - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2018
Income Tax [Line Items]
U.S. Federal statutory income tax rate								21.00%	35.00%
Transition tax		$ (1.7)		$ 76.2				$ (1.7)
Income tax liability payable period	8 years
Aggregate Transition tax installments payments made		10.7						10.7
Noncurrent payable to affiliate	$ 70.1	58.1		70.1		$ 70.1		58.1		$ 70.1
Income tax expense (benefit) related to increase (decrease) in reserve for uncertain tax positions					$ (8.1)			$ 1.4	$ (7.6)
Canada - Germany APA
Income Tax [Line Items]
Advance Pricing Agreement, description								None of our U.S. and non-U.S. tax returns are currently under examination. As a result of prior audits in certain jurisdictions, which are now settled, in 2008 we 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.
Canada - Germany APA | German Subsidiary
Income Tax [Line Items]
Noncash income tax benefit related to APA tax settlement payment			$ 1.4
Canada - Germany APA | Canadian Subsidiary
Income Tax [Line Items]
Income tax expense (benefit) related to increase (decrease) in reserve for uncertain tax positions					(8.1)
Non-cash income tax benefit related to increase in German NOLs					2.6
Cash tax refund					0.6
European and Canadian Subsidiaries
Income Tax [Line Items]
Deferred income tax on undistributed earnings of european and canadian subsidiaries				4.5				$ 2.5
Valhi | Income Tax Payable
Income Tax [Line Items]
Income tax expense paid in current period		$ 4.6						$ 4.6
Payable to affiliate											$ 63.8
Noncurrent payable to affiliate											58.1
Current payable to affiliate											$ 5.7
Valhi | Income Tax Receivable
Income Tax [Line Items]
Prior period income tax expense paid in current fiscal year	6.1			6.1		6.1				6.1
Maximum
Income Tax [Line Items]
Percentage of annual usage limitation on net operating loss carryforward								80.00%
Reversal of Valuation Allowance
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance					(7.8)	(24.1)	$ (149.9)
Effect of Currency Exchange Rates
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance										13.7
Germany | Reversal of Valuation Allowance
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							(141.9)
Germany | Corporate Purposes
Income Tax [Line Items]
Net operating loss carryforwards	652.0			652.0		652.0				652.0
Germany | Trade Tax Purposes
Income Tax [Line Items]
Net operating loss carryforwards	0.5			0.5		0.5				0.5
Belgium | Reversal of Valuation Allowance
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							(8.0)
Belgium | Corporate Purposes
Income Tax [Line Items]
Net operating loss carryforwards	$ 50.0			$ 50.0		$ 50.0				50.0
Germany and Belgian
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance					$ (7.8)		(12.7)			$ (186.7)
Germany and Belgian | Current Periods Net Operating Loss Utilization
Income Tax [Line Items]
Increase (decrease) in non-cash deferred income tax asset valuation allowance							$ (7.7)