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

Gross Book Value of Mortgage Loan

No definition available.

Name:	bxmt_GrossBookValueOfMortgageLoan
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Indicates the number of loans under each classification.

No definition available.

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

The number of loans in maturity default without a provision recorded.

No definition available.

Name:	bxmt_NumberOfLoansMaturityDefault
Namespace Prefix:	bxmt_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Percentage of Loan Loss Reserves

No definition available.

Name:	bxmt_PercentageOfLoanLossReserves
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

Amount of increase (decrease) of allowance reserve for credit loss.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Sale or transfer of a loan during the period that results in a reduction in the excess of a loan's cash flows expected to be collected over the investor's initial investment. This relates to loans accounted for as a held-to-maturity debt security, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 15

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Name:	us-gaap_CertainLoansAcquiredInTransferAccountedForAsHeldToMaturityDebtSecuritiesAccretableYieldDisposalsOfLoans
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.8)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 8

-Article 5

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Data Type:	xbrli:monetaryItemType
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Amount of undisbursed portion of the loan receivable balance (loans in process). Excludes loans and leases covered under loss sharing agreements.

No definition available.

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

Amount of expense related to estimated loss from loan and lease transactions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 11

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.11)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524