Valhi Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	$ 424.4
Other comprehensive income (loss)	(27.2)	$ 42.9	$ (24.9)
Balance at end of year	635.4	424.4
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	1.7	1.7	1.6
Other comprehensive income (loss)			0.1
Balance at end of year	1.7	1.7	1.7
Interest Rate Swap
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year		(1.2)	(1.3)
Other comprehensive income (loss)		(1.2)	(1.2)
Balance at end of year			(1.2)
Less reclassification adjustments for amounts included in realized loss and interest expense		2.4	1.3
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(54.1)	(88.5)	(78.1)
Other comprehensive income (loss)	(21.5)	34.4	(10.4)
Balance at end of year	(75.6)	(54.1)	(88.5)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(129.0)	(137.0)	(123.0)
Other comprehensive income (loss)	7.6	6.4	5.7
Net actuarial gain (loss) arising during the year	(12.6)	1.6	(19.7)
Balance at end of year	(134.0)	(129.0)	(137.0)
Accumulated Defined Benefit Plans Adjustment | OPEB
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	2.4	3.1	3.8
Other comprehensive income (loss)	(0.8)	(0.8)	(1.0)
Net actuarial gain (loss) arising during the year	0.1	0.1	0.3
Balance at end of year	1.7	2.4	3.1
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(179.0)	(221.9)	(197.0)
Balance at end of year	$ (206.2)	$ (179.0)	$ (221.9)