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CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK, REDEEMABLE COMMON STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) (Parenthetical) $ in Thousands | Mar. 31, 2018 USD ($)
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Statement of Stockholders' Equity [Abstract] |
Net of issuance costs | $ 1

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- References No definition available. + Details
Name: | us-gaap_StatementOfStockholdersEquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Preferred Stock Issuance Costs + References No definition available. + Details
Name: | xfor_PreferredStockIssuanceCosts
Namespace Prefix: | xfor_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant