Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Income Tax [Line Items]
Expected tax expense, at U.S. federal statutory income tax rate of 35% in 2017 and 21% in 2018	$ 11.9	$ 26.9	$ 43.1	$ 63.5
Incremental net tax on earnings and losses of non-U.S., U.S. and non-tax group companies	(160.3)	8.9	(151.3)	57.2
Non-U.S. tax rates	3.5	(3.5)	18.4	(8.2)
Valuation allowance	(2.0)	(7.8)		(170.4)
Adjustment to the reserve for uncertain tax positions, net	0.3	(7.7)	2.2	(6.6)
Transition tax	(2.1)		(2.1)
Change in federal tax rate	59.7		59.7		$ (77.1)
Change in state tax rate	(3.4)		(3.4)
Nondeductible expenses	0.8	0.8	1.5	1.5
U.S. state income taxes and other, net	0.2	0.3	0.1	(1.3)
Income tax expense (benefit)	(91.4)	14.7	(33.2)	(67.5)
Comprehensive provision for income taxes (benefit) allocable to:
Income from continuing operations	(91.4)	14.7	(33.2)	(67.5)
Discontinued operations	(0.8)	0.5	19.0	(66.7)
Retained earnings – change in accounting principle			1.1
Other comprehensive income (loss):
Currency translation	0.7	10.6	(1.2)	28.7
Marketable securities		(0.5)		(1.0)
Interest rate swap		1.3		1.4
Total	(90.2)	28.0	(10.2)	(100.8)
Kronos Worldwide, Inc. | Canada ? Germany APA
Schedule Of Income Tax [Line Items]
Canada – Germany APA		(3.2)	(1.4)	(3.2)
Pension Plans
Other comprehensive income (loss):
Defined benefit plans	1.4	1.6	4.4	4.7
OPEB Plans
Other comprehensive income (loss):
Defined benefit plans	$ (0.1)	$ (0.2)	$ (0.3)	$ (0.4)