License Agreement - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 14, 2019	Jan. 31, 2021	Oct. 31, 2020	Nov. 30, 2019	Aug. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Feb. 12, 2019
License Agreement [Line Items]
License agreement, milestone payment due		$ 100,000
Licensing agreement milestone fees	$ 2,000
License agreement milestone payments paid				$ 75,000	$ 2,000
License agreement, additional milestone payment due	102,000
Cost of product sold						$ 14,604	$ 7,890	$ 37,701	$ 17,820
Accrued Sales Based Trademark and Royalties						13,202		13,202		$ 9,006
Accrued milestone payment										100,000
Upon Acceptance by FDA of Pitolisant's
License Agreement [Line Items]
License agreement, milestone payment due											$ 50,000
Upon FDA Approval of WAKIX
License Agreement [Line Items]
License agreement, milestone payment due	77,000
Licensing agreement milestone fees	$ 2,000
License agreement milestone payments paid		100,000	$ 2,000
Sales-based, Trademark and Tiered Royalties
License Agreement [Line Items]
Cost of product sold						13,202	$ 7,297	34,561	$ 16,574
Bioprojet
License Agreement [Line Items]
Accrued milestone payment						$ 0		$ 0		100,000
Bioprojet | United States
License Agreement [Line Items]
Amount of Aggregate Net Sales Attaining										$ 500,000
Bioprojet | Attaining $500,000 Aggregate Net Sales | United States | Upon FDA Approval of WAKIX
License Agreement [Line Items]
License agreement, additional milestone payment due		$ 40,000