Amount of assets pledged to secure a debt instrument.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

Name:	us-gaap_DebtInstrumentCollateralAmount
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 current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
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

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ 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

- Details

Name:	us-gaap_CreditFacilityAxis=bxmt_RevolvingRepurchaseFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_MorganStanleyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_SocieteGeneraleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_JpMorganMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_WellsFargoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_MetlifeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_BankOfAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_CitibankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Secured Debt Agreements - Revolving Repurchase Facilities (Detail) - Revolving Repurchase Facilities [Member]	Jun. 30, 2016 USD ($)	Jun. 30, 2016 GBP (£)	Jun. 30, 2016 EUR (€)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Maximum Facility Size	$ 5,530,725,000			$ 4,332,267,000
Collateral Assets	4,504,794,000			3,811,792,000
Potential	3,511,408,000			2,982,833,000
Repurchase Borrowings Outstanding	3,142,404,000			2,495,805,000
Available	369,004,000			487,028,000
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	335,725,000	£ 250,000,000		370,400,000
Collateral Assets	267,152,000			273,280,000
Potential	210,432,000			212,050,000
Repurchase Borrowings Outstanding	210,432,000			209,038,000
Available				3,012,000
Societe Generale [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	445,000,000		€ 400,000,000	437,320,000
Collateral Assets	166,513,000			67,416,000
Potential	133,211,000			53,933,000
Repurchase Borrowings Outstanding	105,398,000			53,933,000
Available	27,813,000
JP Morgan [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			524,547,000
Collateral Assets	519,014,000			589,752,000
Potential	404,031,000			464,723,000
Repurchase Borrowings Outstanding	393,738,000			382,042,000
Available	10,293,000			82,681,000
Wells Fargo [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	2,000,000,000			1,000,000,000
Collateral Assets	1,421,595,000			879,155,000
Potential	1,099,823,000			687,200,000
Repurchase Borrowings Outstanding	826,255,000			562,382,000
Available	273,568,000			124,818,000
MetLife [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	1,000,000,000			750,000,000
Collateral Assets	946,957,000			593,273,000
Potential	739,102,000			462,849,000
Repurchase Borrowings Outstanding	739,102,000			324,587,000
Available				138,262,000
Bank of America [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	750,000,000			750,000,000
Collateral Assets	649,974,000			840,884,000
Potential	512,679,000			665,861,000
Repurchase Borrowings Outstanding	498,334,000			618,944,000
Available	14,345,000			46,917,000
Citibank [Member]
Line of Credit Facility [Line Items]
Maximum Facility Size	500,000,000			500,000,000
Collateral Assets	533,589,000			568,032,000
Potential	412,130,000			436,217,000
Repurchase Borrowings Outstanding	369,145,000			344,879,000
Available	$ 42,985,000			$ 91,338,000