| Document and Entity Information | Aug. 04, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 4, 2021 |
| Entity Registrant Name | NL Industries, Inc. |
| Entity Incorporation, State or Country Code | NJ |
| Entity File Number | 1-640 |
| Entity Tax Identification Number | 13-5267260 |
| Entity Address, Address Line One | 5430 LBJ Freeway |
| Entity Address, Address Line Two | Suite 1700 |
| Entity Address, City or Town | Dallas |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 75240-2620 |
| City Area Code | 972 |
| Local Phone Number | 233-1700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000072162 |
| Title of 12(b) Security | Common stock |
| Trading Symbol | NL |
| Security Exchange Name | NYSE |
