Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Significant Accounting Policies [Line Items]
Defined Contribution Plan, Cost Recognized	$ 3.0	$ 3.2	$ 3.0
Income tax payments to NL	$ 3.2	$ 5.0	$ 4.3
Minimum | Building
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	15 years
Minimum | Equipment and software
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	3 years
Maximum | Building
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	40 years
Maximum | Equipment and software
Schedule Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives	20 years
NL Industries | CompX International Inc
Schedule Of Significant Accounting Policies [Line Items]
Percentage of ownership held by parent company	86.00%
Valhi Inc | NL Industries
Schedule Of Significant Accounting Policies [Line Items]
Percentage of ownership held by parent company		83.00%
Contran Corporation | Valhi Inc
Schedule Of Significant Accounting Policies [Line Items]
Percentage of ownership held by parent company		92.00%