Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 13, 2020	Jun. 30, 2019
Document and Entity Information [Abstract]
Title of 12(b) Security	Common stock, $0.001 par value per share
Entity Incorporation, State or Country Code	DE
Document Transition Report	false
Document Annual Report	true
Entity Registrant Name	QUALYS, INC.
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
Entity Central Index Key	0001107843
Document Type	10-K
Document Period End Date	Dec. 31, 2019
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		39,092,443
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Public Float			$ 2,505
Entity File Number	001-35662
Entity Tax Identification Number	77-0534145
Trading Symbol	QLYS
Security Exchange Name	NASDAQ
Entity Small Business	false
Entity Emerging Growth Company	false