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

Quarterly report pursuant to Section 13 or 15(d)

Related Party Transactions - Schedule of The Effective Date and Annual Management Services agreement Fee (Income) Expense (Detail)

Related Party Transactions - Schedule of The Effective Date and Annual Management Services agreement Fee (Income) Expense (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Annual Management Services Agreement Fee Income Expense	$ 0
Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	0
Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	0
Helocyte Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Mar. 20, 2015
Helocyte Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Helocyte Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Avenue Therapeutics, Inc. [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Feb. 17, 2015
Avenue Therapeutics, Inc. [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Avenue Therapeutics, Inc. [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Mustang Bio, Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Mar. 13, 2015
Mustang Bio, Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Mustang Bio, Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Checkpoint Therapeutics, Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Mar. 17, 2015
Checkpoint Therapeutics, Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Checkpoint Therapeutics, Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Cellvation Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Oct. 31, 2016
Cellvation Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Cellvation Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Caelum Biosciences, Inc. [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Jan. 01, 2017
Caelum Biosciences, Inc. [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Caelum Biosciences, Inc. [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Cyprium Biosciences, Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Mar. 13, 2017
Cyprium Biosciences, Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Cyprium Biosciences, Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	250
Fortress Biotech Inc [Member]
Annual Management Services Agreement Fee Income Expense	(4,000)
Fortress Biotech Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	(2,000)
Fortress Biotech Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	(2,000)
Aevitas Inc [Member]
Annual Management Services Agreement Fee Income Expense	$ 500
Annual Consulting Fee Payable Effective Date	Jul. 28, 2017
Aevitas Inc [Member] \| Research and Development Expense [Member]
Annual Management Services Agreement Fee Income Expense	$ 250
Aevitas Inc [Member] \| Selling, General and Administrative Expenses [Member]
Annual Management Services Agreement Fee Income Expense	$ 250