Loan Participations Sold, Net - Summary of Statistics for Loan Participations Sold (Detail) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) Loans SecurityLoan
Participating Mortgage Loans [Line Items]
Count \| SecurityLoan	1	1
Principal Balance	$ 106,259	$ 141,119
Weighted Average Term	2022-02	2022-02
Book Value, Total Loan	$ 12,176,516	$ 10,056,732
Book Value	$ 81,044	$ 80,415
Senior Participation [Member]
Participating Mortgage Loans [Line Items]
Count	1	1
Principal Balance	$ 81,170	$ 80,706
Weighted Average Term	2022-02	2022-02
Book Value	$ 81,044	$ 80,415
Loan Participations Sold [Member]
Participating Mortgage Loans [Line Items]
Book Value, Total Loan	$ 105,149	$ 138,907
LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost	6.06%	5.94%
LIBOR [Member] \| Senior Participation [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost	4.08%	4.14%

Number Of Loan Participations Sold

No definition available.

Name:	bxmt_NumberOfLoanParticipationsSold
Namespace Prefix:	bxmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Participating Mortgage Loans Participation Liabilities Principal Amount

+ References

No definition available.

+ Details

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

- Definition

Participations sold loan weighted average term.

+ References

No definition available.

+ Details

Name:	bxmt_ParticipationsSoldLoanWeightedAverageTerm
Namespace Prefix:	bxmt_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Participations Sold Loan Yield

+ References

No definition available.

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

- Definition

The aggregate amount of the participation liability at the end of the accounting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377297&loc=d3e10013-112621

+ Details

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

- Details

Name:	us-gaap_ParticipatingMortgageLoansAxis=bxmt_SeniorParticipationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ParticipatingMortgageLoansAxis=bxmt_LoanParticipationsSoldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: