| Document and Entity Information | Mar. 19, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001107843 |
| Document Type | 8-K |
| Document Period End Date | Mar. 19, 2021 |
| Entity Registrant Name | Qualys, Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35662 |
| Entity Tax Identification Number | 77-0534145 |
| Entity Address, Address Line One | 919 East Hillsdale Boulevard |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.001 par value per share |
| Trading Symbol | QLYS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
