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

Net Loss per Common Share - Computations of Diluted Weighted Average Shares Outstanding (Detail) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	17,667,124	13,063,630
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	1,000,000	0
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	890,892	678,072
Opus warrants to purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	1,880,000	206,593
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	1,085,502	1,095,905
Unvested Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	11,014,596	9,848,505
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total of weighted average shares outstanding	1,796,134	1,234,555