Valhi Stockholders' Equity - Schedule of Common Stock Outstanding (Detail) - shares shares in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Stockholders Equity [Line Items]
Common stock, shares issued	29.6	29.6	29.6
Shares of common stock, Treasury	(1.1)	(1.1)	(1.1)
Common stock, shares outstanding	29.6	29.6
Excluding Stock Ownership By Subsidiary Considered To Be Held In Treasury
Schedule Of Stockholders Equity [Line Items]
Common stock, shares outstanding	28.5	28.5	28.5