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

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

This element represents money paid in advance to protect the provider of a product or service, such as a lessor, against damage or nonpayment by the buyer or tenant (lessee) during the term of the agreement. Such damages may include physical damage to the property, theft of property, and other contractual breaches. Security deposits held may be interest or noninterest bearing.

Reference 1: 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=119400593&loc=d3e572229-122910

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

Aggregate base rent.

No definition available.

Name:	xfor_AggregateBaseRent
Namespace Prefix:	xfor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Current office space under lease agreement.

No definition available.

Name:	xfor_CurrentOfficeSpaceUnderLeaseAgreement
Namespace Prefix:	xfor_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	duration

Lease agreement period of written notice for cancellable.

No definition available.

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

Lessee, Operating Lease, Lease Not Yet Commenced, Amount

No definition available.

Name:	xfor_LesseeOperatingLeaseLeaseNotYetCommencedAmount
Namespace Prefix:	xfor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Lessee, Operating Lease, Lease Not Yet Commenced, Office Space

No definition available.

Name:	xfor_LesseeOperatingLeaseLeaseNotYetCommencedOfficeSpace
Namespace Prefix:	xfor_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	duration

Lessee, Operating Lease, Rent-Free Period

No definition available.

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

Operating lease expiration.

No definition available.

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

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=xfor_CambridgeLeaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=xfor_WalthamLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=xfor_ViennaAustriaLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: