v3.21.2

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended	9 Months Ended
	Mar. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021
Summary Of Significant Accounting Policies [Line Items]
License agreement upfront payment.	$ 10,000
Revenue				$ 0
Aggregate development, regulatory and commercial sales milestone payments	$ 32,500
Collaborative agreements term related to obligation and rights	terminate the agreement prior to expiration in the event of a material breach if not cured within 60 days from the date of notice of such breach (30 days in the case of payment obligations), or either party files for bankruptcy.
Collaborative arrangement proceeds from upfront payment received		$ 5,000	$ 5,000
Reduction of revenue sales returns				2,000
Additional sales return				$ 1,400
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Trade payment term				1 year

Additional Revenue Recognition Sales Returns Reserve For Sales Returns

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Amount of revenue from collaborative arrangement. Excludes revenue from contract with customer under Topic 606.

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Flexion Therapeutics Inc 10-Q