Fortress Biotech, Inc. Annual report pursuant to Section 13 and 15(d)

Collaboration and Stock Purchase Agreements (Narrative) (Details)

Collaboration and Stock Purchase Agreements (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended	12 Months Ended
	Feb. 08, 2019	Jan. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Caelum [Member]
Business Acquisition, Percentage of Voting Interests Acquired		40.00%
Caelum [Member] \| Alexion [Member]
Percentage of shares transferred on a share transaction		19.90%
Sale of Stock, Consideration Received on Transaction		$ 30.0
Initial contingent payments				$ 30.0
Additional upfront funding				20.0
Funding for additional equity interest				$ 60.0
Potential Additional Payments, Maximum		$ 500.0
Sales [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 325.0
Sales After January 1. 2029 [Member] \| Caelum [Member] \| InvaGen [Member]
Gross Profit Percentage To Net Sales			20.00%
Contingent Earn Out Payments Measurement Second Model			$ 1,500.0
Slab One [Member] \| Caelum [Member] \| InvaGen [Member]
Gross Profit Percentage To Net Sales			10.00%
Slab One [Member] \| Sales [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 400.0
Slab One [Member] \| Sales After January 1. 2029 [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 100.0
Slab Two [Member] \| Caelum [Member] \| InvaGen [Member]
Gross Profit Percentage To Net Sales			12.50%
Slab Two [Member] \| Sales [Member] \| Minimum [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 400.0
Slab Two [Member] \| Sales [Member] \| Maximum [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 500.0
Slab Three [Member] \| Caelum [Member] \| InvaGen [Member]
Gross Profit Percentage To Net Sales			15.00%
Slab Three [Member] \| Sales [Member] \| Caelum [Member] \| InvaGen [Member]
Net sales threshold			$ 500.0
SPMA [Member] \| Caelum [Member] \| InvaGen [Member]
Business Combination, possible Interim financing	$ 7.0
Business Combination, possible Interim financing, amount drawn			$ 0.0
First Stage [Member] \| SPMA [Member] \| Avenue [Member] \| InvaGen [Member]
Sale of Stock, Consideration Received on Transaction	$ 35.0
Sale of stock, price per share	$ 6.00
Sale of stock, number of shares issued	5.8
Sale of stock, ownership percentage after the transaction	33.30%