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Merger Accounting - Schedule of Business Acquisitions by Acquisition, Contingent Consideration (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2019 shares
Acquisition Date [Line Items]
Stock issued during period, shares, reverse stock splits	2,440,582
Arsanis [Member]
Acquisition Date [Line Items]
Stock issued during period, shares, reverse stock splits	14,643,737

X

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

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Balance Type:	na
Period Type:	duration

X

- Definition

Acquisition Date [Line Items]

+ References

No definition available.

+ Details

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

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: