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

Quarterly report pursuant to Section 13 or 15(d)

Related Party Transactions - Additional Information (Detail)

Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 17, 2017	May 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Sep. 09, 2016	Mar. 17, 2014
Related Party Transaction [Line Items]
Lease Sharing Agreements Contributions To Property Under Lease					$ 200,000
Equity Method Investment, Ownership Percentage								35.00%
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 4.02		$ 4.02
Avenue Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Payments for Underwriter Fees					$ 2,300,000
Maximum [Member]
Related Party Transaction [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights			4.75		$ 4.75
Minimum [Member]
Related Party Transaction [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 3.61		$ 3.61
National Holdings Corporation [Member]
Related Party Transaction [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired	56.60%		56.60%		56.60%		56.60%
NSC 2017 Note [Member]
Related Party Transaction [Line Items]
Percentage Of Placement Agent Fee	10.00%				10.00%
Warrants on Sale Price of Note,Percentage	10.00%		10.00%		10.00%
Class of Warrant or Right, Number of Securities Called by Warrants or Rights			508,505		508,505
Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party		$ 10,000	$ 28,000		$ 48,000
Board of Directors Chairman [Member] \| National Holdings Corporation [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage			4.60%		4.60%
TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Payments for Rent			$ 1,600,000		$ 500,000
Due from Related Parties, Current			$ 89,000		$ 89,000
Checkpoint [Member]
Related Party Transaction [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired			2.50%		2.50%
Founders Agreement [Member] \| Avenue Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Other Fee,Percentage					2.50%
OPPM [Member]
Related Party Transaction [Line Items]
Payments for Rent					$ 100,000
Due from Related Parties, Current			$ 20,000		20,000
Mustang [Member] \| Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party		7,500
Avenue [Member] \| Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party		$ 5,000
Desk Share Agreements [Member]
Related Party Transaction [Line Items]
Payments for Rent					1,200,000
Desk Share Agreements [Member] \| TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Due from Related Parties, Current			200,000		200,000
Desk Share Agreements [Member] \| OPPM [Member]
Related Party Transaction [Line Items]
Due from Related Parties, Current			200,000		200,000
Shared Services Agreement [Member] \| TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Proceeds from Related Party Agreement			400,000	$ 200,000	600,000	$ 300,000
Due from Related Parties, Current			$ 300,000		$ 300,000