Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Income Tax [Line Items]
United States		$ 23.5	$ (22.5)	$ 26.7
Non-U.S. subsidiaries		81.2	258.7	265.1
Income from continuing operations before income taxes		104.7	236.2	291.8
Expected tax expense at U.S. federal statutory income tax rate of 35% in 2017 and 21% in 2018 and 2019		22.0	49.6	102.1
Non-U.S. tax rates		5.2	20.8	(13.1)
Incremental net tax expense (benefit) on earnings and losses of U.S. and non-U.S. tax group companies		(4.5)	(167.8)	14.8
Valuation allowance		4.5		(205.4)
Transition tax			(2.1)	76.2
Global intangible low-tax income, net		1.8	4.0
Tax rate changes	$ 59.7	4.7	58.8	(77.1)
U.S. state income taxes, net		(0.3)	0.6	3.5
Adjustment to the reserve for uncertain tax positions, net		(5.1)	4.1	(18.2)
Nondeductible expenses		1.5	3.0	2.2
Refund of prior tax payments, net		(2.1)
Domestic production activities deduction				(3.8)
Other, net		(1.2)	(0.3)	(1.2)
Income tax expense (benefit)		26.5	(30.7)	(120.0)
Comprehensive provision for income taxes (benefit) allocable to:
Income tax expense (benefit)		26.5	(30.7)	(120.0)
Discontinued operations			23.7	(67.1)
Retained earnings-change in accounting principle			1.1
Other comprehensive income (loss):
Marketable securities				2.8
Currency translation		(0.2)	(4.2)	31.1
Interest rate swap				1.6
Total		10.2	(15.2)	(143.6)
Pension Plans
Other comprehensive income (loss):
Defined benefit plans		(15.6)	(4.7)	8.6
Other
Other comprehensive income (loss):
Defined benefit plans		$ (0.5)	(0.4)	$ (0.6)
Kronos Worldwide, Inc. | Canada - Germany APA
Schedule Of Income Tax [Line Items]
APA			$ (1.4)