Organization and Basis of Presentation (Policies)	3 Months Ended
Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	Organization. We (NYSE American: CIX) were approximately 87% owned by NL Industries, Inc. (NYSE: NL) at March 31, 2021. We manufacture and sell component products (security products and recreational marine components). At March 31, 2021, Valhi, Inc. (NYSE: VHI) owned approximately 83% of NL’s outstanding common stock and a wholly-owned subsidiary of Contran Corporation owned approximately 92% of Valhi’s outstanding common stock. A majority of Contran's outstanding voting stock is held directly by Lisa K. Simmons and various family trusts established for the benefit of Ms. Simmons, Thomas C. Connelly (the husband of Ms. Simmons’ late sister) and their children and for which Ms. Simmons or Mr. Connelly, as applicable, serve as trustee (collectively, the “Other Trusts”). With respect to the Other Trusts for which Mr. Connelly serves as trustee, he is required to vote the shares of Contran voting stock held in such trusts in the same manner as Ms. Simmons. Such voting rights of Ms. Simmons last through April 22, 2030 and are personal to Ms. Simmons. The remainder of Contran’s outstanding voting stock is held by another trust (the “Family Trust”), which was established for the benefit of Ms. Simmons and her late sister and their children and for which a third-party financial institution serves as trustee. Consequently, at March 31, 2021 Ms. Simmons and the Family Trust may be deemed to control Contran, and therefore may be deemed to indirectly control the wholly-owned subsidiary of Contran, Valhi, NL and us.
Basis of Presentation	Basis of presentation. Consolidated in this Quarterly Report are the results of CompX International Inc. and its subsidiaries. The unaudited Condensed Consolidated Financial Statements contained in this Quarterly Report have been prepared on the same basis as the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2020 that we filed with the Securities and Exchange Commission (“SEC”) on March 3, 2021 (the “2020 Annual Report”). In our opinion, we have made all necessary adjustments (which include only normal recurring adjustments) in order to state fairly, in all material respects, our consolidated financial position, results of operations and cash flows as of the dates and for the periods presented. We have condensed the Consolidated Balance Sheet at December 31, 2020 contained in this Quarterly Report as compared to our audited Consolidated Financial Statements at that date, and we have omitted certain information and footnote disclosures (including those related to the Consolidated Balance Sheet at December 31, 2020) normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Our results of operations for the interim period ended March 31, 2021 may not be indicative of our operating results for the full year. The Condensed Consolidated Financial Statements contained in this Quarterly Report should be read in conjunction with our 2020 Consolidated Financial Statements contained in our 2020 Annual Report. Our operations are reported on a 52 or 53-week year. For presentation purposes, annual and quarterly information in the Condensed Consolidated Financial Statements and accompanying notes are presented as ended March 31, 2020, December 31, 2020 and March 31, 2021. The actual dates of our annual and quarterly periods are March 29, 2020, January 3, 2021 and April 4, 2021, respectively. Unless otherwise indicated, references in this report to “we”, “us” or “our” refer to CompX International Inc. and its subsidiaries, taken as a whole.

Organization and Basis of Presentation (Policies)

3 Months Ended

Mar. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization. We (NYSE American: CIX) were approximately 87% owned by NL Industries, Inc. (NYSE: NL) at March 31, 2021. We manufacture and sell component products (security products and recreational marine components). At March 31, 2021, Valhi, Inc. (NYSE: VHI) owned approximately 83% of NL’s outstanding common stock and a wholly-owned subsidiary of Contran Corporation owned approximately 92% of Valhi’s outstanding common stock. A majority of Contran's outstanding voting stock is held directly by Lisa K. Simmons and various family trusts established for the benefit of Ms. Simmons, Thomas C. Connelly (the husband of Ms. Simmons’ late sister) and their children and for which Ms. Simmons or Mr. Connelly, as applicable, serve as trustee (collectively, the “Other Trusts”). With respect to the Other Trusts for which Mr. Connelly serves as trustee, he is required to vote the shares of Contran voting stock held in such trusts in the same manner as Ms. Simmons. Such voting rights of Ms. Simmons last through April 22, 2030 and are personal to Ms. Simmons. The remainder of Contran’s outstanding voting stock is held by another trust (the “Family Trust”), which was established for the benefit of Ms. Simmons and her late sister and their children and for which a third-party financial institution serves as trustee. Consequently, at March 31, 2021 Ms. Simmons and the Family Trust may be deemed to control Contran, and therefore may be deemed to indirectly control the wholly-owned subsidiary of Contran, Valhi, NL and us.

Basis of Presentation

Basis of presentation. Consolidated in this Quarterly Report are the results of CompX International Inc. and its subsidiaries. The unaudited Condensed Consolidated Financial Statements contained in this Quarterly Report have been prepared on the same basis as the audited Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2020 that we filed with the Securities and Exchange Commission (“SEC”) on March 3, 2021 (the “2020 Annual Report”). In our opinion, we have made all necessary adjustments (which include only normal recurring adjustments) in order to state fairly, in all material respects, our consolidated financial position, results of operations and cash flows as of the dates and for the periods presented. We have condensed the Consolidated Balance Sheet at December 31, 2020 contained in this Quarterly Report as compared to our audited Consolidated Financial Statements at that date, and we have omitted certain information and footnote disclosures (including those related to the Consolidated Balance Sheet at December 31, 2020) normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Our results of operations for the interim period ended March 31, 2021 may not be indicative of our operating results for the full year. The Condensed Consolidated Financial Statements contained in this Quarterly Report should be read in conjunction with our 2020 Consolidated Financial Statements contained in our 2020 Annual Report.

Our operations are reported on a 52 or 53-week year. For presentation purposes, annual and quarterly information in the Condensed Consolidated Financial Statements and accompanying notes are presented as ended March 31, 2020, December 31, 2020 and March 31, 2021. The actual dates of our annual and quarterly periods are March 29, 2020, January 3, 2021 and April 4, 2021, respectively. Unless otherwise indicated, references in this report to “we”, “us” or “our” refer to CompX International Inc. and its subsidiaries, taken as a whole.