Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Jul. 31, 2021
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Registrant Name	ZAI LAB LIMITED
Entity Central Index Key	0001704292
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Interactive Data Current	Yes
Entity File Number	001-38205
Entity Incorporation, State or Country Code	E9
Entity Address, Address Line One	4560 Jinke Road
Entity Address, Address Line Two	Bldg. 1, Fourth Floor Pudong
Entity Address, City or Town	Shanghai
Entity Address, Country	CN
Entity Address, Postal Zip Code	201210
Document Quarterly Report	true
Document Transition Report	false
Entity Tax Identification Number	98-1144595
Entity Small Business	false
Local Phone Number	6163 2588
City Area Code	86 21
American Depositary Shares
Document Information [Line Items]
Trading Symbol	ZLAB
Entity Common Stock, Shares Outstanding		65,826,281
Title of 12(b) Security	American Depositary Shares, each representing 1 Ordinary Share, par value $0.00006 per share
Security Exchange Name	NASDAQ
Ordinary Shares
Document Information [Line Items]
Trading Symbol	9688
Entity Common Stock, Shares Outstanding		95,408,743
Title of 12(b) Security	Ordinary Shares, par value $0.00006 per share