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

Fair Value Measurements - Warrant Liabilities And Convertible Notes At Fair Value (Detail)

Fair Value Measurements - Warrant Liabilities And Convertible Notes At Fair Value (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.00%
Helocyte [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Dividend Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.00%
Helocyte [Member] \| Convertible Debt [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility		52.40%
Helocyte [Member] \| Minimum [Member] \| Convertible Debt [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		1.53%
Helocyte [Member] \| Minimum [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		6 months
Helocyte [Member] \| Maximum [Member] \| Convertible Debt [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		1.72%
Helocyte [Member] \| Maximum [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		10 months 28 days
Caelum Convertible Notes [Member] \| Convertible Debt [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate	2.302%
Caelum Convertible Notes [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Dividend Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.00%
Caelum Convertible Notes [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years	3 months 25 days
Caelum Convertible Notes [Member] \| Convertible Debt [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility	67.00%	70.00%
Caelum Convertible Notes [Member] \| Minimum [Member] \| Convertible Debt [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		1.506%
Caelum Convertible Notes [Member] \| Minimum [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		5 months 16 days
Caelum Convertible Notes [Member] \| Maximum [Member] \| Convertible Debt [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		1.851%
Caelum Convertible Notes [Member] \| Maximum [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		1 year 8 months 12 days