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

Collaboration and Stock Purchase Agreements - Additional Information (Details)

Collaboration and Stock Purchase Agreements - Additional Information (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Feb. 08, 2019	Jan. 30, 2019	Dec. 31, 2019	Jan. 31, 2019	Nov. 12, 2018
Business Acquisition, Percentage of Voting Interests Acquired					33.30%
Business Combination, Contingent Consideration, Liability					$ 180.0
Caelum [Member]
Sale of Stock Percentage of Shares Transferred on Transaction		19.90%
Sale of Stock, Consideration Received on Transaction		$ 30.0
Business Acquisition, Percentage of Voting Interests Acquired				40.00%
Alexion [Member]
Potential Additional Payments, Maximum		$ 500.0
Sales [Member] \| Avenue [Member]
Net sales threshold			$ 325.0
Sales After January 1. 2029 [Member] \| Avenue [Member]
Gross Profit Percentage To Net Sales			20.00%
Contingent Earn Out Payments Measurement Second Model			$ 1,500.0
Slab One [Member] \| Avenue [Member]
Gross Profit Percentage To Net Sales			10.00%
Slab One [Member] \| Sales [Member] \| Avenue [Member]
Net sales threshold			$ 400.0
Slab One [Member] \| Sales After January 1. 2029 [Member] \| Avenue [Member]
Net sales threshold			$ 100.0
Slab Two [Member] \| Avenue [Member]
Gross Profit Percentage To Net Sales			12.50%
Slab Two [Member] \| Sales [Member] \| Minimum
Net sales threshold			$ 400.0
Slab Two [Member] \| Sales [Member] \| Maximum
Net sales threshold			$ 500.0
Slab Three [Member] \| Avenue [Member]
Gross Profit Percentage To Net Sales			15.00%
Slab Three [Member] \| Sales [Member] \| Avenue [Member]
Net sales threshold			$ 500.0
SPMA [Member]
Business Combination, Consideration Transferred	$ 7.0
First Stage [Member] \| SPMA [Member]
Business Combination, Consideration Transferred	$ 31.5