Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 18, 2020
Cover [Abstract]
Entity Registrant Name	Xenon Pharmaceuticals Inc.
Document Type	10-Q/A
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		34,974,119
Amendment Flag	true
Amendment Description	Xenon Pharmaceuticals Inc. (the “Company,” “we” or “us”) is filing this Quarterly Report on Form 10-Q for the three months ended March 31, 2020 (this “Form 10-Q”) in reliance upon the relief set forth in the Order (the “Order”) issued by the Securities and Exchange Commission (the “Commission”) on March 25, 2020 (Release No. 34-88465) under Section 36 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). The Order provides conditional relief to public companies that are unable to timely comply with certain filing obligations as a result of the novel coronavirus (“COVID-19”) outbreak. This Form 10-Q could not be filed within the time period specified under the Exchange Act, absent the relief available under the Order, due to the outbreak and spread of COVID-19. Specifically, employees in the Company’s financial reporting and legal departments have been working remotely and therefore been unable to maintain the same ordinary course interactions with the Company’s professional advisors. As a result, the preparation and completion of the Company’s financial statements for the three months ended March 31, 2020 to be included in this Form 10-Q was delayed.Pursuant to the requirements of the Order, we filed a Form 8-K with the Commission on May 6, 2020 indicating our intention to rely upon the Order with respect to the filing of this Form 10-Q, which would have otherwise been required to have been filed by May 11, 2020. This Form 10-Q is being filed within the 45-day extension period provided by the Order.
Entity Central Index Key	0001582313
Entity Filer Category	Accelerated Filer
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Small Business	true
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity File Number	001-36687
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
Title of 12(b) Security	Common Shares, without par value
Trading Symbol	XENE
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	Z4
Document Quarterly Report	true
Document Transition Report	false