Secured Debt Agreements, Net - Summary of Asset-Specific Financings (Parenthetical) (Detail) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Participating Mortgage Loans [Line Items]
Borrowings	$ 1,538,548,000	$ 518,864,000
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Borrowings	1,538,548,000	518,864,000
GE Portfolio Acquisition Facility [Member]
Participating Mortgage Loans [Line Items]
Borrowings	510,405,000	703,423,000
Maximum Facility Size	510,400,000	816,300,000
Approximate credit line, outstanding amount	81,300,000	703,400,000
Amount available for potential future fundings of loan	$ 112,900,000
GE Portfolio Acquisition Facility [Member] \| Minimum [Member]
Participating Mortgage Loans [Line Items]
Percentage of guarantee rate	25.00%
Guarantee of outstanding borrowings	$ 250,000,000
Cross Collateral Assets [Member] \| Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Borrowings	$ 551,700,000	$ 394,800,000

Amount of guarantee obligations.

No definition available.

Name:	bxmt_AmountOfGuaranteeObligations
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of recourse debt.

+ References

No definition available.

+ Details

Name:	bxmt_PercentageOfRecourseDebt
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Secured Debt Repurchase Agreements Face Amount

+ References

No definition available.

+ Details

Name:	bxmt_SecuredDebtRepurchaseAgreementsFaceAmount
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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

- 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

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

- Details

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

- Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=bxmt_CrossCollateralAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: