Consolidated Securitization Vehicles (Narrative) (Details) (Securitization Vehicles, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Maximum exposure to loss	$ 6,600
Number of loans in maturity default	1
Book value of loans in maturity default	27,000
CT CDOs
Loans receivable, CT CDOs	$ 141,300	$ 141,500

Net book value of loans in consolidated securitization vehicles which were in maturity default but had not provision recorded as of the balance sheet date.

No definition available.

Name:	bxmt_BookValueOfLoansInMaturityDefault
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant