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Commitments and Contingencies - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Directors
Loans
Commitments And Contingencies [Line Items]  
Unfunded commitments $ 513,200,000bxmt_UnfundedCommitmentsRelatedToLoansReceivable
Commitments expiration period 5 years
Number of loans receivable 35bxmt_NumberOfLoansReceivableOutstanding
Number of board of directors 8bxmt_NumberOfDirectors
Number of directors entitled to annual compensation 5bxmt_NumberOfDirectorsEligibleForAnnualCompensation
Pending Litigation [Member]  
Commitments And Contingencies [Line Items]  
Reserves recorded for pending litigation 0us-gaap_LitigationReserve
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
Five Board of Directors [Member]  
Commitments And Contingencies [Line Items]  
Annual cash compensation 125,000us-gaap_NoninterestExpenseDirectorsFees
/ us-gaap_TitleOfIndividualAxis
= bxmt_BoardOfDirectorsMember
Three Board of Directors [Member]  
Commitments And Contingencies [Line Items]  
Annual cash compensation 0us-gaap_NoninterestExpenseDirectorsFees
/ us-gaap_TitleOfIndividualAxis
= bxmt_ThreeMembersOfBoardOfDirectorsMember
Four Directors [Member]  
Commitments And Contingencies [Line Items]  
Percentage of compensation paid in cash 50.00%bxmt_PercentageOfCompensationPaidInCash
/ us-gaap_TitleOfIndividualAxis
= bxmt_FourDirectorsMember
Percentage of compensation in the form of deferred stock units 50.00%bxmt_PercentageOfCompensationFormOfDeferredStockUnits
/ us-gaap_TitleOfIndividualAxis
= bxmt_FourDirectorsMember
Chairperson of Audit Committee [Member]  
Commitments And Contingencies [Line Items]  
Annual cash compensation $ 12,000us-gaap_NoninterestExpenseDirectorsFees
/ us-gaap_TitleOfIndividualAxis
= bxmt_AuditDirectorMember