Organization and Basis of Presentation (Policies)	3 Months Ended
Organization and Basis of Presentation (Policies)	Mar. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	Organization. We (NYSE American: CIX) are 87% owned by NL Industries, Inc. (NYSE: NL) at March 31, 2018. We manufacture and sell component products (security products and recreational marine components). At March 31, 2018, Valhi, Inc. (NYSE: VHI) owns 83% of NL’s outstanding common stock and a wholly-owned subsidiary of Contran Corporation owns 93% of Valhi’s outstanding common stock. All of Contran’s outstanding voting stock is held by a family trust established for the benefit of Lisa K. Simmons and Serena Simmons Connelly and their children, for which Ms. Simmons and Ms. Connelly are co-trustees, or is held directly by Ms. Simmons and Ms. Connelly or entities related to them. Consequently, Ms. Simmons and Ms. Connelly may be deemed to control 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, 2017 that we filed with the Securities and Exchange Commission (“SEC”) on February 28, 2018 (the “2017 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, 2017 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, 2017) 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, 2018 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 2017 Consolidated Financial Statements contained in our 2017 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, 2017, December 31, 2017 and March 31, 2018. The actual dates of our annual and quarterly periods are April 2, 2017, December 31, 2017 and April 1, 2018, 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.
Revenue Recognition	Revenue Recognition Our sales involve single performance obligations to ship our products pursuant to customer purchase orders. In some cases, the purchase order is supported by an underlying master sales agreement, but our purchase order verification notice generally evidences the contract with our customer by specifying the key terms of product and quantity ordered, price and delivery and payment terms. Effective January 1, 2018 with the adoption of Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606)(see Note 10), we record revenue when we satisfy our performance obligations to our customers by transferring control of our products to them, which generally occurs at point of shipment or upon delivery. Such transfer of control is also evidenced by transfer of legal title and other risks and rewards of ownership (giving the customer the ability to direct the use of, and obtain substantially all of the benefits of, the product), and our customers becoming obligated to pay us and such payment being probable of occurring. In certain arrangements we provide shipping and handling activities after the transfer of control to our customer (e.g. when control transfers prior to delivery). In such arrangements shipping and handling are considered fulfillment activities, and accordingly, such costs are accrued when the related revenue is recognized. Revenue is recorded in an amount that reflects the net consideration we expect to receive in exchange for our products. Prices for our products are based on terms specified in published list prices and purchase orders, which generally do not include financing components, noncash consideration or consideration paid to our customers. As our standard payment terms are less than one year, we have elected the practical expedient under ASC 606 and we have not assessed whether a contract has a significant financing component. We state sales net of price, early payment and distributor discounts as well as volume rebates (collectively, variable consideration). Variable consideration, to the extent present, is not material and is recognized as the amount to which we are most-likely to be entitled, using all information (historical, current and forecasted) that is reasonably available to us, and only to the extent that a significant reversal in the amount of the cumulative revenue recognized is not probable of occurring in a future period. Differences, if any, between estimates of the amount of variable consideration to which we will be entitled and the actual amount of such variable consideration have not been material in the past. We report any tax assessed by a governmental authority that we collect from our customers that is both imposed on and concurrent with our revenue-producing activities (such as sales, use, value added and excise taxes) on a net basis (meaning we do not recognize these taxes either in our revenues or in our costs and expenses). Frequently, we receive orders for products to be delivered over dates that may extend across reporting periods. We invoice for each delivery upon shipment and recognize revenue for each distinct shipment when all sales recognition criteria for that shipment have been satisfied. As scheduled delivery dates for these orders are within a one year period, under the optional exemption provided by ASC 606, we do not disclose sales allocated to future shipments of partially completed contracts. ASU 2014-09 requires a disaggregation of our sales into categories that depict how the nature, amount, timing and uncertainty of revenue and cash flows are affected by economic factors. We have determined such disaggregation of our sales is the same as the disclosure of our sales by segment. See Note 2.