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Discontinued Operations - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 0 Months Ended
Dec. 31, 2012
Dec. 19, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Purchase agreement, purchase price $ 21,424us-gaap_ProceedsFromDivestitureOfBusinesses  
CT Legacy REIT [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Sale of outstanding class A preferred stock   100.00%us-gaap_SaleOfStockPercentageOfOwnershipBeforeTransaction
/ dei_LegalEntityAxis
= bxmt_Subsidiaries1Member
Sale to Holdings III [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Purchase agreement, purchase price   21,400us-gaap_ProceedsFromDivestitureOfBusinesses
/ us-gaap_SubsidiarySaleOfStockAxis
= bxmt_SaleToHoldingsMember