Term Loans, Net - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2021 USD ($)
Debt Instrument [Line Items]
Secured term loan percentage of partially amortizing	1.00%
B-1 Term Loan
Debt Instrument [Line Items]
Increase in borrowings	$ 200.0
Secured term loan percentage of partially amortizing	1.00%
Discount upon issuance of secured term loan	$ 3.1
Secured term loan transaction expenses	$ 12.6
Total debt to total assets ratio	0.8333
B-1 Term Loan \| LIBOR
Debt Instrument [Line Items]
Basis spread on debt obligation	2.25%
B-2 Term Loan
Debt Instrument [Line Items]
Increase in borrowings	$ 100.0
Basis spread on debt obligation	0.50%
Secured term loan percentage of partially amortizing	1.00%
Discount upon issuance of secured term loan	$ 9.6
Secured term loan transaction expenses	$ 5.4
Total debt to total assets ratio	0.8333
B-2 Term Loan \| LIBOR
Debt Instrument [Line Items]
Decrease in basis spread on debt obligation	2.50%
Basis spread on debt obligation	2.75%

Represents debt instrument amortization percentage.

No definition available.

Name:	bxmt_DebtInstrumentAmortizationPercentage
Namespace Prefix:	bxmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Covenant, Total Debt to Total Assets, Maximum

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentCovenantTotalDebtToTotalAssetsMaximum
Namespace Prefix:	bxmt_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument gross financing amount.

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Increase (Decrease) in Basis Spread on Variable Rate

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentIncreaseDecreaseInBasisSpreadOnVariableRate
Namespace Prefix:	bxmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types: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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=124435984&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

+ Details

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

- Definition

Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=124435984&loc=d3e28555-108399

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=bxmt_TwentyNineteenTermLoanMember
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:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=bxmt_TwentyTwentyTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: