Common Stock and Preferred Stock (Converted into Common Stock) - Preferred Stock (Details) € / shares in Units, $ / shares in Units, $ in Thousands, € in Millions	Sep. 30, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2018 EUR (€) € / shares shares
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)	0	0
Preferred stock, par value (in USD per share) | $ / shares	$ 0.0001	$ 0.0001
Preferred stock issued (in shares) | $	$ 0	$ 0
Preferred stock, shares authorized (in shares)	20,000,000	20,000,000
Preferred stock, shares outstanding (in shares)	0	0
Immunic AG
Class of Stock [Line Items]
Preferred stock issued (in shares)			$ 37,200	€ 31.7
Immunic AG | Series A-1 Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			13,541	13,541
Preferred stock, par value (in USD per share) | € / shares				€ 1.00
Immunic AG | Series A-2 Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			299,456	299,456