Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Income Tax [Line Items]
United States			$ (22.5)	$ 26.7	$ (8.2)
Non-U.S. subsidiaries			258.7	265.1	47.8
Income from continuing operations before income taxes			236.2	291.8	39.6
Expected tax expense (benefit) at U.S. federal statutory income tax rate of 35% in 2016 and 2017 and 21% in 2018			49.6	102.1	13.8
Non-U.S. tax rates			20.8	(13.1)	(4.3)
Incremental net tax expense (benefit) on earnings and losses of non-U.S. and non-tax group companies			(167.8)	14.8	8.2
Valuation allowance				(205.4)	(2.2)
Transition tax			(2.1)	76.2
Global intangible low-tax income, net			4.0
Change in federal tax rate	$ 59.7		60.6	(77.1)
Change in state tax rate			(1.8)
U.S. state income taxes, net			0.6	3.5	1.7
Adjustment to the reserve for uncertain tax positions, net			4.1	(18.2)	7.2
Nondeductible expenses			3.0	2.2	1.9
Domestic production activities deduction				(3.8)	(3.8)
Other, net			(0.3)	(1.2)	(0.5)
Provision for income taxes (benefit)			(30.7)	(120.0)	18.6
Comprehensive provision for income taxes (benefit) allocable to:
Income (loss) from continuing operations			(30.7)	(120.0)	18.6
Discontinued operations			23.7	(67.1)	(12.6)
Retained earnings-change in accounting principle			1.1
Other comprehensive income (loss):
Marketable securities				2.8	2.1
Currency translation			(4.2)	31.1	(3.4)
Interest rate swap				1.6	0.2
Total			(15.2)	(143.6)	(0.6)
Pension Plans
Other comprehensive income (loss):
Defined benefit plans			(4.7)	8.6	(5.0)
Other
Other comprehensive income (loss):
Defined benefit plans			(0.4)	$ (0.6)	(0.5)
Canada - Germany APA
Schedule Of Income Tax [Line Items]
APA		$ (11.8)
Kronos Worldwide, Inc. | Canada - Germany APA
Schedule Of Income Tax [Line Items]
APA			$ (1.4)
Kronos Worldwide, Inc. | US-Canada APA
Schedule Of Income Tax [Line Items]
APA					$ (3.4)