Financing Receivable, Commitment to Lend

No definition available.

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

- Definition

Financing Receivable, Number of Loans

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Percentage Of Loan Portfolio

+ References

No definition available.

+ Details

Name:	bxmt_FinancingReceivablePercentageOfLoanPortfolio
Namespace Prefix:	bxmt_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Mortgage loan on real estate final maturity term.

+ References

No definition available.

+ Details

Name:	bxmt_MortgageLoanOnRealEstateFinalMaturityTerm
Namespace Prefix:	bxmt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted Average Cash Coupon Rate

+ References

No definition available.

+ Details

Name:	bxmt_WeightedAverageCashCouponRate
Namespace Prefix:	bxmt_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, fee, and loan in process, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481598/310-20-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479853/942-210-S99-1

+ Details

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

- Definition

Yield on the receivable, on which interest has been imputed, as calculated from its issuance value or purchase price. The calculated effective interest rate considers factors such as the issued face value or price paid for the receivable, the time period between payments, and the time until maturity [full receipt] of the receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482900/835-30-50-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482925/835-30-45-2

+ Details

Name:	us-gaap_ReceivableWithImputedInterestEffectiveYieldInterestRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_UnfundedLoanCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bxmt_PrepaymentRestrictionsIncludingYieldMaintenanceLockOutProvisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bxmt_WithoutPrepaymentRestrictionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	3 Months Ended				12 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2024 USD ($) security_loan	Mar. 31, 2024 USD ($)	Mar. 31, 2024 USD ($) loan	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) security_loan	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans		173		173		178		178
Principal balance	$ 23,317,882	$ 23,317,882	$ 23,317,882	$ 23,317,882	$ 23,923,719	$ 23,923,719	$ 23,923,719	$ 23,923,719
Net book value	$ 22,437,942	$ 22,437,942	$ 22,437,942	$ 22,437,942	$ 23,210,076	$ 23,210,076	$ 23,210,076	$ 23,210,076
Weighted-average maximum maturity (years)	2 years 2 months 12 days				2 years 4 months 24 days
Percentage of portfolio	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
Interest Rate Swaps
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notional amount	$ 229,900	$ 229,900	$ 229,900	$ 229,900	$ 229,900	$ 229,900	$ 229,900	$ 229,900
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded loan commitments	$ 2,089,678	$ 2,089,678	$ 2,089,678	$ 2,089,678	$ 2,430,664	$ 2,430,664	$ 2,430,664	$ 2,430,664
Prepayment Restrictions Including Yield Maintenance Lock Out Provisions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	13.00%	13.00%	13.00%	13.00%	14.00%	14.00%	14.00%	14.00%
Without Prepayment Restrictions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of portfolio	87.00%	87.00%	87.00%	87.00%	86.00%	86.00%	86.00%	86.00%
Secured Overnight Financing Rate (SOFR)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon (percentage)			3.36%				3.37%
Weighted-average all-in yield (percentage)			3.68%				3.71%