| Document and Entity Information | Aug. 04, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 4, 2020 |
| Entity Registrant Name | CompX International Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 1-13905 |
| Entity Tax Identification Number | 57-0981653 |
| 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 | 448-1400 |
| 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 | 0001049606 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | CIX |
| Security Exchange Name | NYSEAMER |
