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Variable Interest Entities - Additional Information (Detail) - loan	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Variable Interest Entity, Primary Beneficiary, Does Not Hold Majority Voting Interest, Disclosures [Abstract]
Number of loans modified	2	2

X

- Definition

Financing Receivable, Number Of Loans Modified

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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