Revenue - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2020
Disaggregation Of Revenue [Line Items]
Revenue	$ 8,124,000	$ 6,554,000	$ 14,700,000	$ 27,016,000
Accounts receivable	3,235,000		3,235,000			$ 1,822,000
Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Accounts receivable	2,052,000		2,052,000
Deferred revenue	2,202,000		2,202,000
Neurocrine Biosciences [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Milestone payments	5,270,000		5,270,000
Neurocrine and Flexion [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,854,000	6,554,000	6,430,000	27,016,000
Flexion [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Milestone payments			3,000,000
Exclusive License to XEN901 and Exclusive License to DTCs [Member] | Neurocrine Biosciences [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		4,898,000		22,182,000
Development Services Under the Initial Development Program For DTCs [Member] | Neurocrine Biosciences [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	823,000	348,000	1,440,000	824,000
Research And Development Funding | Neurocrine Biosciences [Member] | Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,031,000	$ 1,308,000	4,990,000	$ 4,010,000
License and Collaboration Agreement [Member] | Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Upfront payment received in cash					$ 30,000,000
Upfront payment received in equity investment					20,000,000
Equity investment measured at fair value					16,667,000
Collaborate agreement premium related to equity investment					3,333,000
Transaction price allocated to performance obligations					33,333,000
Variable consideration allocated to performance obligation			592,000
Aggregate milestone payment received			10,000,000
Milestone payment received in cash			4,500,000
Milestone payment received in equity investment			5,500,000
Milestone payment received in equity investment measured at fair value			4,730,000
Collaborate milestone agreement premium related to equity investment			770,000
Revenue recognized milestone cash payment			4,500,000
Aggregate price of transaction allocated to performance obligations related to milestone			$ 0
Percentage of option to co-fund development costs upon achievement of certain milestones			50.00%
License and Collaboration Agreement [Member] | Exclusive License to XEN901 and Exclusive License to DTCs [Member] | Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Transaction price allocated to performance obligations					$ 28,807,000
License and Collaboration Agreement [Member] | Development Services Under the Initial Development Program For DTCs [Member] | Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Transaction price allocated to performance obligations			$ 5,118,000
Asset Purchase Agreement [Member] | Flexion [Member] | XEN402 [Member]
Disaggregation Of Revenue [Line Items]
Milestone payments			1,000,000
Milestone payment due on initiation of Phase 1b clinical trial			2,000,000
Asset Purchase Agreement [Member] | Eligible to Receive Development Milestone Payment [Member] | Flexion [Member] | XEN402 [Member] | Maximum [Member]
Disaggregation Of Revenue [Line Items]
Potential milestone payments receivable	5,000,000		5,000,000
Asset Purchase Agreement [Member] | Future Clinical Development And Global Regulatory Approval Milestone Payments [Member] | Flexion [Member] | XEN402 [Member] | Maximum [Member]
Disaggregation Of Revenue [Line Items]
Potential milestone payments receivable	40,750,000		40,750,000
Asset Purchase Agreement [Member] | Commercial Milestone Payments [Member] | Flexion [Member] | XEN402 [Member] | Maximum [Member]
Disaggregation Of Revenue [Line Items]
Potential milestone payments receivable	$ 75,000,000		$ 75,000,000