Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15(a))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

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

- Definition

Minimum amount of purchase arrangement maturing in the second fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

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

- Definition

Minimum amount of purchase arrangement maturing in the remainder of the fiscal year following the latest fiscal year ended. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

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

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ Details

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

- Definition

Contingencies And Commitments [Line Items]

+ References

No definition available.

+ Details

Name:	xfor_ContingenciesAndCommitmentsLineItems
Namespace Prefix:	xfor_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Funding commitment period.

+ References

No definition available.

+ Details

Name:	xfor_FundingCommitmentPeriod
Namespace Prefix:	xfor_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum funding commitment amount.

+ References

No definition available.

+ Details

Name:	xfor_MaximumFundingCommitmentAmount
Namespace Prefix:	xfor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Milestone payments.

+ References

No definition available.

+ Details

Name:	xfor_MilestonePayments
Namespace Prefix:	xfor_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Option fee.

+ References

No definition available.

+ Details

Name:	xfor_OptionFee
Namespace Prefix:	xfor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_TypeOfArrangementAxis=xfor_SponsoredResearchAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=xfor_IndemnificationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=xfor_AdimabOptionAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=xfor_ContractManufacturingOrganizationsAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitment and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended
	Apr. 30, 2017	Feb. 28, 2017	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Contingencies And Commitments [Line Items]
Research and development expense			$ 5,655,000	$ 4,744,000
Sponsored Research Agreement [Member]
Contingencies And Commitments [Line Items]
Purchase commitment, amount	$ 499,000
Purchase commitment, period	3 years
Research and development expense			42,000	$ 42,000
Non-cancelable purchase commitments amount			181,000
Non-cancelable purchase commitments amount, 2019			125,000
Non-cancelable purchase commitments amount, 2020			56,000
Indemnification Agreements [Member]
Contingencies And Commitments [Line Items]
Accrued liabilities			0		$ 0
Adimab Option Agreement [Member]
Contingencies And Commitments [Line Items]
Milestone payments		$ 25,000,000
Option fee		$ 250,000
Contract Manufacturing Organizations Agreement [Member]
Contingencies And Commitments [Line Items]
Non-cancelable purchase commitments amount			$ 250,000		$ 250,000