Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Aug. 31, 2016	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Note receivable from affiliate		$ 30,500,000		$ 28,100,000
Unsecured Revolving Demand Promissory | Valhi Inc
Related Party Transaction [Line Items]
Maximum loan amount	$ 40,000,000
Interest rate on loans repayment	1.00%
Principal due on demand effective date	Dec. 31, 2021
Note receivable from affiliate		30,500,000
Interest income including unused commitment fees on our loan		$ 1,100,000	$ 1,900,000