| Document and Entity Information | Apr. 20, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | Zai Lab Ltd |
| Amendment Flag | false |
| Entity Central Index Key | 0001704292 |
| Document Type | 8-K |
| Document Period End Date | Apr. 20, 2021 |
| Entity Incorporation State Country Code | E9 |
| Entity File Number | 001-38205 |
| Entity Tax Identification Number | 98-1144595 |
| Entity Address, Address Line One | 4560 Jinke Road |
| Entity Address, Address Line Two | Bldg. 1 |
| Entity Address, Address Line Three | Fourth Floor Pudong |
| Entity Address, City or Town | Shanghai |
| Entity Address, Country | CN |
| Entity Address, Postal Zip Code | 201210 |
| City Area Code | 86 21 |
| Local Phone Number | 6163 2588 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | American Depository Shares, each representing 1 Ordinary Share, par value $0.00006 per share |
| Trading Symbol | ZLAB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
