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.

No definition available.

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

+ Details

Name:	us-gaap_DerivativeNumberOfInstrumentsHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
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:	us-gaap_DerivativeInstrumentRiskAxis=bxmt_ForeignExchangeContractBuyGreatBritainPoundSellUnitedStatesDollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

Derivative Financial Instruments - Summary of Non-designated Hedges (Detail) € in Thousands, £ in Thousands, $ in Thousands	Sep. 30, 2019 EUR (€) DerivativeInstrument	Dec. 31, 2018 GBP (£) DerivativeInstrument	Dec. 31, 2018 EUR (€) DerivativeInstrument	Dec. 31, 2018 AUD ($) DerivativeInstrument
AUD [Member] \| Buy AUD / Sell USD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amount \| $				$ 55,000
AUD [Member] \| Buy USD / Sell AUD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amount \| $				$ 55,000
EUR [Member] \| Buy GBP / Sell EUR Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Notional Amount \| €	€ 12,857
Not Designated as Hedging Instrument [Member] \| AUD [Member] \| Buy AUD / Sell USD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments		1	1	1
Not Designated as Hedging Instrument [Member] \| AUD [Member] \| Buy USD / Sell AUD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments		1	1	1
Not Designated as Hedging Instrument [Member] \| GBP [Member] \| Buy GBP / Sell USD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments		1	1	1
Notional Amount \| £		£ 23,200
Not Designated as Hedging Instrument [Member] \| GBP [Member] \| Buy USD / Sell GBP Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments		1	1	1
Notional Amount \| £		£ 23,200
Not Designated as Hedging Instrument [Member] \| EUR [Member] \| Buy GBP / Sell EUR Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments	1	1	1	1
Notional Amount \| €			€ 12,857
Not Designated as Hedging Instrument [Member] \| EUR [Member] \| Buy EUR / Sell USD Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments	1
Notional Amount \| €	€ 131,900
Not Designated as Hedging Instrument [Member] \| EUR [Member] \| Buy USD / Sell EUR Forward [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of Instruments	1
Notional Amount \| €	€ 131,900