Foreign currency derivatives cancelled

No definition available.

Name:	bxmt_ForeignCurrencyDerivativesCancelled
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

+ Details

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

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

- Definition

The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80845-113994

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_GBP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_AUD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_CAD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments - Additional Information (Detail) € in Millions, £ in Millions, $ in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Jun. 30, 2020 EUR (€)	Jun. 30, 2020 GBP (£)	Jun. 30, 2020 AUD ($)	Jun. 30, 2020 CAD ($)	Mar. 31, 2020 EUR (€)	Mar. 31, 2020 GBP (£)	Mar. 31, 2020 AUD ($)	Mar. 31, 2020 CAD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of collateral posted for the net assets/liability positions									$ 51,050,000	$ 30,800,000
Interest Rate Swaps/Derivatives [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Reclassification from accumulated other comprehensive income (loss) as increase to interest income									$ 10,000
Foreign Currency Contracts [Member] \| EUR [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Foreign currency derivatives cancelled \| €	€ 620.4				€ 552.1
Foreign Currency Contracts [Member] \| GBP [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Foreign currency derivatives cancelled \| £		£ 530.2				£ 365.5
Foreign Currency Contracts [Member] \| AUD [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Foreign currency derivatives cancelled			$ 92.8				$ 134.8
Foreign Currency Contracts [Member] \| CAD [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Foreign currency derivatives cancelled				$ 24.4				$ 23.7