Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 18, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	001-35366
Entity Registrant Name	Fortress Biotech, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-5157386
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Address, Address Line One	2 Gansevoort Street
Entity Address, Address Line Two	9th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10014
City Area Code	781
Local Phone Number	652-4500
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 173,878,853
Entity Central Index Key	0001429260
Document Fiscal Year Focus	2020
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Common Stock [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock
Trading Symbol	FBIO
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		94,907,448
9.375% Series A Cumulative Redeemable Perpetual Preferred Stock [Member]
Document Information [Line Items]
Title of 12(b) Security	9.375% Series A Cumulative Redeemable Perpetual Preferred Stock
Trading Symbol	FBIOP
Security Exchange Name	NASDAQ
Entity Common Stock, Shares Outstanding		3,427,138