| Document and Entity Information | Oct. 05, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 5, 2021 |
| Entity Registrant Name | XENON PHARMACEUTICALS INC. |
| Entity Central Index Key | 0001582313 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-36687 |
| Entity Incorporation, State or Country Code | Z4 |
| Entity Tax Identification Number | 98-0661854 |
| Entity Address, Address Line One | 200-3650 Gilmore Way |
| Entity Address, City or Town | Burnaby |
| Entity Address, State or Province | BC |
| Entity Address, Country | CA |
| Entity Address, Postal Zip Code | V5G 4W8 |
| City Area Code | 604 |
| Local Phone Number | 484-3300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Shares, without par value |
| Trading Symbol | XENE |
| Security Exchange Name | NASDAQ |
