Variable Interest Entities - Summary of Assets and Liabilities of Consolidated CLO and Single Asset Securitization VIE (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Assets	$ 2,700,000	$ 1,400,000
VIE [Member]
Assets:
Assets	2,709,533	1,401,691
Liabilities:
Liabilities	2,241,726	1,188,732
VIE [Member] \| Loans Receivable [Member]
Assets:
Assets	2,717,763	1,349,903
Loans receivable, net	2,702,656	1,349,903
VIE [Member] \| Current Expected Credit Loss Reserve [Member]
Assets:
Assets	(15,107)
VIE [Member] \| Other Assets [Member]
Assets:
Assets	6,877	51,788
VIE [Member] \| Securitized Debt Obligations, Net [Member]
Liabilities:
Liabilities	2,239,640	1,187,084
VIE [Member] \| Other Liabilities [Member]
Liabilities:
Liabilities	$ 2,086	$ 1,648

Variable Interest Entity Consolidated Carrying Amount, Assets Net

No definition available.

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Balance Type:	debit
Period Type:	instant

- Definition

Variable Interest Entity Consolidated Carrying Amount, Current Expected Credit Loss reserve.

+ References

No definition available.

+ Details

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Namespace Prefix:	bxmt_
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Balance Type:	credit
Period Type:	instant

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5710-111685

+ Details

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- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Amount of consolidated variable interest entity's (VIE) liabilities that do not have recourse to general credit of primary beneficiary.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116870748&loc=SL6758485-165988

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5710-111685

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- References

No definition available.

+ Details

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- Details

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