Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 28, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Trading Symbol	CIX
Entity Registrant Name	COMPX INTERNATIONAL INC
Entity Central Index Key	0001049606
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		12,451,157
Document Quarterly Report	true
Document Transition Report	false
City Area Code	972
Entity Address, Address Line One	5430 LBJ Freeway
Entity Address, Address Line Two	Suite 1700
Entity Address, Address Line Three	Three Lincoln Centre
Entity Address, City or Town	Dallas
Entity Address, Postal Zip Code	75240
Entity Address, State or Province	TX
Entity Current Reporting Status	Yes
Entity File Number	1-13905
Entity Incorporation, State or Country Code	DE
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Tax Identification Number	57-0981653
Local Phone Number	448-1400
Title of 12(b) Security	Class A common stock
Security Exchange Name	NYSEAMER