Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2023	Apr. 28, 2023
Document Information [Line Items]
Entity Central Index Key	0001107843
Entity Registrant Name	QUALYS, INC.
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	2023
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2023
Document Transition Report	false
Entity File Number	001-35662
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	77-0534145
Entity Address, Address Line One	919 E. Hillsdale Boulevard, 4th Floor,
Entity Address, City or Town	Foster City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94404
City Area Code	650
Local Phone Number	801-6100
Title of 12(b) Security	Common stock, $0.001 par value per share
Trading Symbol	QLYS
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,895,797