Fair Value of Financial Instruments Additional Information (Details)	6 Months Ended
Jun. 30, 2020 security
Fair Value Disclosures [Abstract]
Marketable Securities, Securities In Unrealized Loss Positions, Qualitative Disclosure, Number Of Positions	0

Marketable Securities, Securities In Unrealized Loss Positions more than 12 months, Qualitative Disclosure, Number Of Positions

No definition available.

Name:	qlys_MarketableSecuritiesSecuritiesInUnrealizedLossPositionsMoreThan12MonthsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	qlys_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration