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Segment Reporting - Additional Information (Detail) (Geographic Concentration Risk [Member], International Sources [Member], Sales [Member])
12 Months Ended
Dec. 31, 2014
Geographic Concentration Risk [Member] | International Sources [Member] | Sales [Member]
 
Segment Reporting Information [Line Items]  
Percentage of revenues generated from non-domestic sources 11.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_GeographicConcentrationRiskMember
/ us-gaap_StatementGeographicalAxis
= bxmt_ForeignCustomersMember