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

Debt and Interest- Additional Information (Detail)

v3.10.0.1

Debt and Interest- Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 12, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt [Line Items]
Payments of Financing Costs			$ 404	$ 1,081
Opus Credit Facility [Member]
Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			12.00%
Debt instrument Expiration Period	Sep. 14, 2019				Sep. 14, 2018
Venture Notes [Member]
Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		8.00%
Debt Instrument, Face Amount		$ 21,700
Debt Instrument, Term			36 months
Debt Instrument, Maturity Date, Description			The notes mature 36 months from issuance, provided that during the first 24 months the Company may extend the maturity date by six months.
NSC [Member] \| Venture Notes [Member]
Debt [Line Items]
Warrant On Sale Price Of Note Percentage			25.00%
Payments of Financing Costs		$ 1,700
Class of Warrant or Right, Expiration Period			10 years
Aevitas Therapeutics, Inc [Member] \| Venture Notes [Member]
Debt [Line Items]
Proceeds from Notes Payable			$ 1,800
Tamid Bio, Inc [Member] \| Venture Notes [Member]
Debt [Line Items]
Proceeds from Notes Payable			1,100
Cellvation [Member] \| Venture Notes [Member]
Debt [Line Items]
Proceeds from Notes Payable			1,000
Cyprium [Member] \| Venture Notes [Member]
Debt [Line Items]
Proceeds from Notes Payable			$ 1,200

- Definition

The expiration period of warrants or rights.

+ References

No definition available.

+ Details

Name:	fbio_ClassOfWarrantOrRightExpirationPeriod
Namespace Prefix:	fbio_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Expiration Period ofDebtinstrument.

+ References

No definition available.

+ Details

Name:	fbio_DebtInstrumentExpirationPeriod
Namespace Prefix:	fbio_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	fbio_DebtLineItems
Namespace Prefix:	fbio_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

It represents the percentage rate of warrants which are issued on sale price of notes.

+ References

No definition available.

+ Details

Name:	fbio_WarrantOnSalePriceOfNotePercentage
Namespace Prefix:	fbio_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentMaturityDateDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for loan and debt issuance costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfFinancingCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from a borrowing supported by a written promise to pay an obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromNotesPayable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=fbio_OpusCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=fbio_VentureNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fbio_NSCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fbio_AevitasTherapeuticsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fbio_TamidBioIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fbio_CellvationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=fbio_CypriumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: