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

Fair Value Measurements - Option pricing model (Detail)

Fair Value Measurements - Option pricing model (Detail)	12 Months Ended
Fair Value Measurements - Option pricing model (Detail)	Dec. 31, 2019	Dec. 31, 2018
Caelum [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility	70.00%
Caelum [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 [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 [Member] \| Convertible Debt [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		3 months 26 days
Caelum [Member] \| Convertible Debt [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility		67.00%
Caelum [Member] \| Warrants [Member] \| Measurement Input, Risk Free Interest Rate [Member] \| Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		2.905%
Caelum [Member] \| Warrants [Member] \| Measurement Input, Risk Free Interest Rate [Member] \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate		2.909%
Caelum [Member] \| Warrants [Member] \| Measurement Input, Expected Dividend Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield		0.00%
Caelum [Member] \| Warrants [Member] \| Measurement Input, Expected Term [Member] \| Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		3 years 10 months 2 days
Caelum [Member] \| Warrants [Member] \| Measurement Input, Expected Term [Member] \| Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years		3 years 11 months 16 days
Caelum [Member] \| Warrants [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility		70.00%
Cyprium [Member] \| Warrants [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Probability of issuance of the warrant	5.00%	0.00%
Cyprium [Member] \| Warrants [Member] \| Measurement Input, Risk Free Interest Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Risk-free interest rate	1.92%	0.00%
Cyprium [Member] \| Warrants [Member] \| Measurement Input, Expected Dividend Rate [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected dividend yield	0.00%	0.00%
Cyprium [Member] \| Warrants [Member] \| Measurement Input, Expected Term [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected term in years	10 years	0 years
Cyprium [Member] \| Warrants [Member] \| Measurement Input, Price Volatility [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected volatility	93.00%	0.00%