Debt Obligations - Repurchase Obligations Outstanding (Detail) (Repurchase Agreements [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 1,614,525
Principal Balance	1,411,355
Net Book Value	1,401,937
Potential	1,082,177
Current	863,622
Available	218,555
Asset-Specific Repurchase Agreements [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,902,879
Principal Balance	1,746,212
Net Book Value	1,735,355
Potential	1,327,908
Current	1,109,353
Available	218,555
Bank of America [Member] \| Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000
Principal Balance	355,981
Net Book Value	352,995
Potential	280,500
Current	271,320
Available	9,180
Citibank [Member] \| Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000
Principal Balance	613,339
Net Book Value	609,236
Potential	460,765
Current	334,692
Available	126,073
JP Morgan [Member] \| Revolving Repurchase Facilities [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	614,525
Principal Balance	442,035
Net Book Value	439,706
Potential	340,912
Current	257,610
Available	83,302
Wells Fargo [Member] \| Asset-Specific Repurchase Agreements [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	288,354
Principal Balance	334,857
Net Book Value	333,418
Potential	245,731
Current	245,731
Available

Repurchase Facility Maximum Borrowing Capacity

No definition available.

Name:	bxmt_RepurchaseFacilityMaximumBorrowingCapacity
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the aggregate value of loans, trade receivables, securities and financial instruments that serve as collateral for obligations arising from a transfer of financial assets recorded as a secured borrowing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5086-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7523297&loc=SL6224234-111729

+ Details

Name:	us-gaap_LoansAndLeasesReceivableCollateralForSecuredBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=7523297&loc=SL6224234-111729

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 10

-Section 05

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=7517284&loc=d3e93645-111703

+ Details

Name:	us-gaap_SecuritiesReceivedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant