Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 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	$ 22.4	$ 14.7
Incremental net tax on earnings and losses of non-U.S. and non-tax group companies	4.1	11.2
Non-U.S. tax rates	7.0	(2.4)
Valuation allowance	0.3	(5.0)
Adjustment to the reserve for uncertain tax positions, net	1.6	0.5
Nondeductible expenses	0.4	0.4
Domestic production activities deduction		(0.6)
U.S. state income taxes and other, net	1.8	(0.2)
Income tax expense	36.2	18.6
Comprehensive provision for income taxes (benefit) allocable to:
Income from continuing operations	36.2	18.6
Discontinued operations	20.1	(0.8)
Retained earnings – change in accounting principle	1.1
Other comprehensive income (loss):
Currency translation	1.4	1.8
Interest rate swap		0.4
Marketable securities		(0.1)
Total	60.2	20.7
Kronos Worldwide, Inc. | Canada – Germany APA
Schedule Of Income Tax [Line Items]
Canada – Germany APA	(1.4)
Pension Plans
Other comprehensive income (loss):
Defined benefit plans	1.5	0.9
OPEB Plans
Other comprehensive income (loss):
Defined benefit plans	$ (0.1)	$ (0.1)