License and Other Agreements - AstraZeneca Agreement (Details) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) shares	May 31, 2017 shares	Oct. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2017 USD ($)
License and other agreements
Value of common stock issued				$ 282,833	$ 21,221	$ 281,100
Payment for license agreement							$ 41,500	$ 0
Issuance of common shares as payment for assets					$ 5,646
AstraZeneca | License Agreement
License and other agreements
Payment for license agreement	$ 3,000
Issuance of common shares as payment for assets (in shares) | shares	109,523
Issuance of common shares as payment for assets	$ 4,080
Milestone payment due upon achievement of specified regulatory and commercial milestones	55,000
Milestone payment due upon achievement of specified sales-based milestones	$ 50,000
Agreement time period	10 years
AstraZeneca | Collaborative arrangement
License and other agreements
Expiration period based on country by country basis after first commercial sale (in years)			10 years
Common stock value agreed to issue			$ 10,000
Value of common stock issued			$ 5,000
Percentage of shares to be issued upon closing of the transaction			0.50
Percentage of remaining shares to be issued after contractual events			0.50
Fair value of common share | $ / shares			$ 9.2911
Preferred stock issued and sold (in shares) | shares			538,150
Aggregate fair value			$ 5,000
Reclassifications of contingent equity to carrying value			5,000
Common stock issued (shares) | shares		538,150
Change in fair value of contingent equity liability									$ 4,273
AstraZeneca | Collaborative arrangement | Maximum
License and other agreements
Milestone payment to be paid upon regulatory achievement			30,000
Milestone payment to be paid for any product other than rimegepant			60,000
Commercial milestone payment to be paid			$ 120,000
Right to purchase shares fully diluted (as a percent)			0.08
AstraZeneca | Collaborative arrangement | Minimum
License and other agreements
Minimum investment in equity			$ 30,000