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Equity - Additional Information (Detail) (USD $)
 0 Months Ended 3 Months Ended 0 Months Ended
Mar. 16, 2015
Apr. 15, 2014
Mar. 14, 2014
Mar. 31, 2015
Mar. 31, 2014
Apr. 15, 2015
May 09, 2014
Dec. 31, 2014
Apr. 08, 2015
May 31, 2013
Mar. 25, 2014
Class of Stock [Line Items]                      
Total stock, shares authorized       200,000,000bxmt_StockSharesAuthorized              
Common stock, shares authorized       100,000,000us-gaap_CommonStockSharesAuthorized       100,000,000us-gaap_CommonStockSharesAuthorized      
Preferred stock, shares authorized       100,000,000us-gaap_PreferredStockSharesAuthorized              
Preferred stock issued       0us-gaap_PreferredStockSharesIssued              
Preferred stock outstanding       0us-gaap_PreferredStockSharesOutstanding              
Plan adoption date       March 25, 2014              
Common stock issued       58,451,077us-gaap_CommonStockSharesIssued       58,269,889us-gaap_CommonStockSharesIssued      
Number of shares sold during period       139us-gaap_StockIssuedDuringPeriodSharesNewIssues 9,775,000us-gaap_StockIssuedDuringPeriodSharesNewIssues            
Sale of common stock sales price       $ 585,000us-gaap_CommonStockValue       $ 583,000us-gaap_CommonStockValue      
Dividends paid   18,900,000us-gaap_PaymentsOfDividendsCommonStock   30,300,000us-gaap_PaymentsOfDividendsCommonStock 13,276,000us-gaap_PaymentsOfDividendsCommonStock            
Dividends declared per common stock $ 0.52us-gaap_CommonStockDividendsPerShareDeclared   $ 0.48us-gaap_CommonStockDividendsPerShareDeclared $ 0.52us-gaap_CommonStockDividendsPerShareDeclared $ 0.48us-gaap_CommonStockDividendsPerShareDeclared            
Dividends paid per common stock   $ 0.48us-gaap_CommonStockDividendsPerShareCashPaid                  
Date of dividend paid       Apr. 15, 2015 Apr. 15, 2014            
Date of dividend declared       Mar. 16, 2015 Mar. 14, 2014            
Record date of dividend paid       Mar. 31, 2015 Mar. 31, 2014            
Accumulated other comprehensive loss       (31,756,000)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax       (15,024,000)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax      
CT Legacy Partners' total equity       78,400,000us-gaap_LimitedLiabilityCompanyLlcMembersEquityIncludingPortionAttributableToNoncontrollingInterest              
Equity interests owned by Blackstone Mortgage Trust, Inc.       32,600,000us-gaap_MembersEquity              
Non-controlling interests in CT Legacy Partners       45,800,000us-gaap_MembersEquityAttributableToNoncontrollingInterest              
Subsequent Events [Member]                      
Class of Stock [Line Items]                      
Total stock, shares authorized                 300,000,000bxmt_StockSharesAuthorized
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		    
Dividends paid           30,400,000us-gaap_PaymentsOfDividendsCommonStock
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		          
Dividends paid per common stock           $ 0.52us-gaap_CommonStockDividendsPerShareCashPaid
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		          
Class A Common Stock [Member]                      
Class of Stock [Line Items]                      
Common stock, shares authorized       100,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		              
Common stock issued                   25,875,000us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		  
Aggregate sales price             200,000,000bxmt_AggregateSalesPriceOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		        
Class A Common Stock [Member] | Subsequent Events [Member]                      
Class of Stock [Line Items]                      
Common stock, shares authorized                 200,000,000us-gaap_CommonStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		    
Class A Common Stock [Member] | ATM Agreements [Member]                      
Class of Stock [Line Items]                      
Number of shares sold during period       0us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_SubsidiarySaleOfStockAxis
= bxmt_AtTheMarketAgreementMember		              
Sale of common stock sales price       $ 197,200,000us-gaap_CommonStockValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
/ us-gaap_SubsidiarySaleOfStockAxis
= bxmt_AtTheMarketAgreementMember		              
Class A Common Stock [Member] | Direct Stock Purchase Plan [Member]                      
Class of Stock [Line Items]                      
Common stock issued       0us-gaap_CommonStockSharesIssued
/ us-gaap_PlanNameAxis
= bxmt_DirectStockPurchasePlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		              
Class A Common Stock [Member] | Dividend Reinvestment Plan [Member]                      
Class of Stock [Line Items]                      
Common stock issued       139us-gaap_CommonStockSharesIssued
/ us-gaap_PlanNameAxis
= bxmt_DividendReinvestmentPlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		              
Class A Common Stock [Member] | Dividend Reinvestment and Direct Stock Purchase Plan [Member]                      
Class of Stock [Line Items]                      
Reserved for issuance of class A common stock       9,999,855us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= bxmt_DividendReinvestmentAndDirectStockPurchasePlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		             10,000,000us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= bxmt_DividendReinvestmentAndDirectStockPurchasePlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember