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Equity - Schedule of Primary Components of Accumulated Other Comprehensive Loss (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2014
Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning   $ (15,024)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ 798us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Amortization of net deferred gains on settlement of swaps (56)us-gaap_AmortizationOfDeferredHedgeGains    
Deconsolidation of subsidiaries 29,889us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation    
Balance, ending   (15,024)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax 798us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Mark-to-Market on Interest Rate Hedges [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (27,423)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
   
Unrealized gain on derivative financial instruments 8,367us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
   
Ineffective portion of cash flow hedges 2,481bxmt_GainLossOnCashFlowHedgeIneffectivenessReclassificationFromOtherComprehensiveIncome
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
   
Deconsolidation of subsidiaries 16,575us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
   
Deferred Gains on Settled Hedges [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning 56us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= bxmt_AccumulatedDeferredGainsHedgesMember
   
Amortization of net deferred gains on settlement of swaps (56)us-gaap_AmortizationOfDeferredHedgeGains
/ us-gaap_StatementEquityComponentsAxis
= bxmt_AccumulatedDeferredGainsHedgesMember
   
Other-Than-Temporary Impairments [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (16,578)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
   
Other-than-temporary impairments of securities 678us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxIncludingPortionAttributableToNoncontrollingInterest
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
   
Deconsolidation of subsidiaries 15,900us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
   
Unrealized Gains on Securities [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning 3,361us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
   
Amortization of net unrealized gains on securities (775)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
   
Deconsolidation of subsidiaries (2,586)us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
   
Accumulated Other Comprehensive Income (loss) [Member]      
Accumulated Other Comprehensive Income (Loss) [Line Items]      
Balance, beginning (40,584)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Unrealized gain on derivative financial instruments 8,367us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Ineffective portion of cash flow hedges 2,481bxmt_GainLossOnCashFlowHedgeIneffectivenessReclassificationFromOtherComprehensiveIncome
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Amortization of net unrealized gains on securities (775)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Amortization of net deferred gains on settlement of swaps (56)us-gaap_AmortizationOfDeferredHedgeGains
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Other-than-temporary impairments of securities 678us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxIncludingPortionAttributableToNoncontrollingInterest
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
   
Deconsolidation of subsidiaries $ 29,889us-gaap_NoncontrollingInterestDecreaseFromDeconsolidation
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember