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Revenue - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Revenue		$ 6,554	$ 3,500	$ 27,016	$ 3,500
Accounts receivable		1,609		1,609		$ 813
Neurocrine Biosciences [Member]
Disaggregation Of Revenue [Line Items]
Accounts receivable		1,382		1,382
Deferred revenue		7,446		7,446
Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		6,554	3,500	27,016	3,500
Exclusive License to XEN901 and Exclusive License to DTCs [Member] \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		4,898		22,182
Development Services Under the Initial Development Program For DTCs [Member] \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		348		824
Research And Development Funding \| Neurocrine Biosciences [Member] \| Collaboration Revenue [Member]
Disaggregation Of Revenue [Line Items]
Revenue		$ 1,308		$ 4,010
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
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						$ 4,526
Asset Purchase Agreement [Member] \| Flexion [Member] \| XEN402 [Member]
Disaggregation Of Revenue [Line Items]
Proceeds from upfront fee	$ 3,000
Asset Purchase Agreement [Member] \| Flexion [Member] \| XEN402 [Member] \| ASC 606 [Member]
Disaggregation Of Revenue [Line Items]
Potential milestone payments receivable	500
Asset Purchase Agreement [Member] \| Eligible to Receive CMC Development and Regulatory Milestone Payments [Member] \| Flexion [Member] \| XEN402 [Member] \| Maximum [Member]
Disaggregation Of Revenue [Line Items]
Potential milestone payments receivable	9,000		9,000		9,000
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		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		$ 75,000