Fair Value (Details Narrative) (Securitization Vehicles, Subordinated interests in mortgages, USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Coupon or Weighted Average Coupon	3.80%
Hotel
Principal Balance	18,100
Number of loans - reported at fair value	2

Indicates the number of loans under each classification reported at fair value.

No definition available.

Name:	bxmt_NumberOfLoansReportedAtFairValue
Namespace Prefix:	bxmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant