XML 53 R37.htm IDEA: XBRL DOCUMENT v3.19.1

Nature of the Business and Basis of Presentation - Additional Information (Detail) $ in Millions		3 Months Ended
	May 14, 2019 USD ($)	Mar. 31, 2019	Mar. 13, 2019
Schedule Of Accounting Policies [Line Items]
Percentage of common stock owned			63.70%
Net proceeds from sale of common stock and pre-funded warrants	$ 79
Arsanis [Member]
Schedule Of Accounting Policies [Line Items]
Business acquisition share exchange ratio		0.5702

X

- Definition

Percentage of voting equity interests acquired at the acquisition date in the business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

+ Details

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

X

- Definition

Business Combination Share Exchange Ratio1

+ References

No definition available.

+ Details

Name:	xfor_BusinessCombinationShareExchangeRatio1
Namespace Prefix:	xfor_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Net Proceeds From Issuance Of Common Stock And Warrants

+ References

No definition available.

+ Details

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

X

- Definition

Schedule Of Accounting Policies [Line Items]

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_BusinessAcquisitionAxis=xfor_ArsanisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: