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Revenue - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenue	$ 2,218	$ 13,384	$ 6,576	$ 20,462
Accounts receivable	2,919		2,919		$ 1,822
Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	2,218	13,384	3,576	20,462
Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Accounts receivable	1,624		1,624
Deferred revenue	3,025		3,025
Flexion [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Milestone payments			3,000
Exclusive License to XEN901 and Exclusive License to DTCs [Member] \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		11,523		17,285
Development Services Under the Initial Development Program For DTCs [Member] \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	617	393	617	475
Research And Development Funding \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue	1,601	$ 1,468	2,959	$ 2,702
License and Collaboration Agreement [Member] \| Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Upfront payment received in cash						$ 30,000
Upfront payment received in equity investment						20,000
Equity investment measured at fair value						16,667
Collaborate agreement premium related to equity investment						3,333
Transaction price allocated to performance obligations						33,333
Variable consideration allocated to performance obligation			$ 592
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
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
Asset Purchase Agreement [Member] \| Flexion [Member]
Disaggregation Of Revenue [Line Items]
Milestone payment due on initiation of Phase 1b clinical trial			$ 2,000
Asset Purchase Agreement [Member] \| Flexion [Member] \| XEN402 [Member]
Disaggregation Of Revenue [Line Items]
Milestone payments			1,000
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		5,000
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		40,750
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		$ 75,000