Disclosure details of loan receivable portfolio based on floor rate [Line Items].

No definition available.

Name:	bxmt_DisclosureDetailsOfLoanReceivablePortfolioBasedOnFloorRateLineItems
Namespace Prefix:	bxmt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Floor rate percentage on loan porfolio excluding zero point zero percentage.

+ References

No definition available.

+ Details

Name:	bxmt_FloorRatePercentageOnLoanPorfolioExcludingZeroPointZeroPercentage
Namespace Prefix:	bxmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Floor rate percentage on loan portfolio.

+ References

No definition available.

+ Details

Name:	bxmt_FloorRatePercentageOnLoanPortfolio
Namespace Prefix:	bxmt_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Mortgage loans on real estate accounted for under cost recovery method.

+ References

No definition available.

+ Details

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

- Definition

Single asset securitization amount subordinate position.

+ References

No definition available.

+ Details

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

- Definition

This is the principal amount outstanding for securitized loans only (across all types of loans).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=d3e107314-111719

+ Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=bxmt_TwoThousandAndEighteenSingleAssetSecuritizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bxmt_IndexRateFloorAxis=bxmt_ZeroPercentOrNoFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bxmt_IndexRateFloorAxis=bxmt_ZeroPointZeroOnePercentToZeroPointTwoFourPercentFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
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:	bxmt_IndexRateFloorAxis=bxmt_ZeroPointTwoFivePercentToZeroPointNineNinePercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	bxmt_IndexRateFloorAxis=bxmt_OnePercentOrMoreFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans Receivable, Net - Schedule Of Loan Receivable Portfolio Based On Floor Rate (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Mortgage loans on real estate accounted for under cost recovery method	$ 338.7
2018 Single Asset Securitization [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Principal amount outstanding of loans	493.3	$ 735.5
Single asset securitization principal amount subordinate position	$ 79.2
0.00% or no floor
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.00%
0.01% to 0.24% floor \| Maximum [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.24%
0.01% to 0.24% floor \| Minimum [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.01%
0.25% to 0.99% floor \| Maximum [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.99%
0.25% to 0.99% floor \| Minimum [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.25%
0.25% to 0.99% floor \| Weighted Average [Member]
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	0.55%
Floor rate percentage on loan portfolio excluding zero point zero percentage	0.89%
1.00% or more floor
Disclosure Details Of Loan Receivable Portfolio Based On Floor Rate [Line Items]
Floor rate percentage on loan portfolio	1.00%