Fortress Biotech, Inc. Quarterly report pursuant to Section 13 or 15(d)

Quarterly report pursuant to Section 13 or 15(d)

Collaboration and Stock Purchase Agreements (Narrative) (Details)

Collaboration and Stock Purchase Agreements (Narrative) (Details) $ / shares in Units, $ in Thousands, shares in Millions				3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 06, 2021 USD ($)	Feb. 24, 2021 USD ($) Milestone	Feb. 08, 2019 USD ($) $ / shares shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Collaboration revenue				$ 1,446	$ 0	$ 4,646	$ 0
Accumulated deficit				(515,898)		(515,898)		$ (482,760)
Cash and cash equivalents				252,721	$ 218,389	252,721	$ 218,389	233,351
Cyprium [Member] \| Sentynl Therapeutics, Inc [Member]
Upfront fees payment		$ 8,000
Payments of milestones		$ 12,000
Percentage of ownership over any FDA priority review voucher		100.00%
Number of net sales milestones \| Milestone		5
Number of development milestones \| Milestone		2
Collaboration revenue				1,400		4,600		$ 0
Avenue [Member]
Accumulated deficit				76,100		76,100
Cash and cash equivalents				$ 600		$ 600
SPMA [Member] \| Minimum [Member] \| Avenue [Member] \| InvaGen [Member]
Percentage of common shares acquired in the Stock Purchase Transaction for the rights to exist			75.00%
FDA approval of the NDA [Member] \| Cyprium [Member] \| Sentynl Therapeutics, Inc [Member]
Payments of milestones		$ 9,000
Achievement of Certain Sales Milestones [Member] \| Maximum [Member] \| Cyprium [Member] \| Sentynl Therapeutics, Inc [Member]
Payments of milestones		255,000
Strategic Transaction, First Stage [Member] \| SPMA [Member] \| Avenue [Member] \| InvaGen [Member]
Sale of stock, number of shares issued \| shares			5.8
Stock offering, price per share \| $ / shares			$ 6.00
Sale of Stock, Consideration Received on Transaction			$ 35,000
Sale of stock, ownership percentage after the transaction			33.30%
Strategic Transaction, Second Stage [Member] \| SPMA [Member] \| Avenue [Member] \| InvaGen [Member]
Sale of Stock, Consideration Received on Transaction			$ 180,000
Completion of Clinical Development Milestones [Member] \| Cyprium [Member] \| Sentynl Therapeutics, Inc [Member]
Payments of milestones		$ 3,000
Subsequent Events [Member] \| Caelum [Member] \| AstraZeneca [Member]
Deconsolidation of consolidated partner company, upfront payment	$ 150,000
Deconsolidation of consolidated partner company, contingent payment	350,000
Subsequent Events [Member] \| Maximum [Member] \| Caelum [Member] \| AstraZeneca [Member]
Deconsolidation of consolidated partner company, contingent payment	$ 350,000