Document and Entity Information	12 Months Ended
	Dec. 31, 2018 shares
Document And Entity Information [Abstract]
Document Type	20-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	ZLAB
Entity Registrant Name	Zai Lab Ltd
Entity Central Index Key	0001704292
Entity Common Stock, Shares Outstanding	58,355,903
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false