Investment in Kronos Worldwide, Inc. - Additional Information (Detail) - Kronos - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Number of shares owned	35.2	35.2
Aggregate market value	$ 572.3	$ 907.6
Quoted market price per share	$ 16.25	$ 25.77

Balance held at close of period in number of shares.

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