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Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Parenthetical) (Detail)
 12 Months Ended
Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]  
Commitments expiration period 5 years
Percentage of loans subject to yield maintenance, lock-out provisions, or other prepayment restrictions 85.00%bxmt_PercentageOfLoansSubjectToYieldMaintenanceLockOutProvisionsOtherPrepaymentRestrictions
Percentage of loans open to repayment by borrower 15.00%bxmt_PercentageOfLoansOpenToRepaymentByBorrower
LIBOR [Member]  
Accounts, Notes, Loans and Financing Receivable [Line Items]  
Percentage of loans receivable by type 14.00%bxmt_PercentageOfLoansReceivableByType
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
Average floor rate 0.31%bxmt_LiborAverageFloorRate
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
Unfunded Commitments [Member]  
Accounts, Notes, Loans and Financing Receivable [Line Items]  
Commitments expiration period 4 years