| Document and Entity Information | Apr. 18, 2022 |
| Document And Entity Information [Line Items] | |
| Entity Registrant Name | Zai Lab Ltd |
| Amendment Flag | true |
| Entity Central Index Key | 0001704292 |
| Document Type | 8-K/A |
| Document Period End Date | Apr. 18, 2022 |
| 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 |
| Entity Emerging Growth Company | false |
| Amendment Description | This Amendment No. 2 on the Current Report on Form 8-K/A amends the Current Report on Form 8-K dated April 18, 2022 (the “Form 8-K”) by disclosing that the Company has entered into an engagement letter with KPMG LLP on May 31, 2022 to be our independent registered public accounting firm, effective May 31, 2022. For convenience, the entire text of the Form 8-K regarding the change in our certifying accountant is provided below, reflecting the relevant updates. |
| Adr [Member] | |
| Document And Entity Information [Line Items] | |
| Security Exchange Name | NASDAQ |
| Security 12b Title | American Depositary Shares, each representing 10 Ordinary Shares, par value $0.000006 per share |
| Trading Symbol | ZLAB |
| Fair Value Measured at Net Asset Value Per Share [Member] | |
| Document And Entity Information [Line Items] | |
| Security Exchange Name | CHX |
| Security 12b Title | Ordinary Shares, par value $0.000006 per share* |
| Trading Symbol | 9688 |
