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Merger Accounting - Schedule of Business Acquisitions by Acquisition, Contingent Consideration (Parenthetical) (Detail) | 3 Months Ended
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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

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- 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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Period Type: | duration

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- 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

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |