Secured Financings - Summary of Key Terms of Revolving Repurchase Facilities (Detail)	3 Months Ended
Mar. 31, 2015
Wells Fargo [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+1.82 %
Rate	1.82%
Guarantee	25.00%
Advance Rate	79.18%
Margin Call	Collateral marks only
Term/Maturity	Term matched
JP Morgan [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+1.87 %
Rate	1.87%
Guarantee	25.00%
Advance Rate	80.09%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Bank of America [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+1.80 %
Rate	1.80%
Guarantee	50.00%
Advance Rate	79.54%
Margin Call	Collateral marks only
Term/Maturity	May 21, 2019
MetLife [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+1.81 %
Rate	1.81%
Guarantee	50.00%
Advance Rate	77.82%
Margin Call	Collateral marks only
Term/Maturity	February 24, 2021
Citibank [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+1.92 %
Rate	1.92%
Guarantee	25.00%
Advance Rate	76.64%
Margin Call	Collateral marks only
Term/Maturity	Term matched
Morgan Stanley [Member]
Line of Credit Facility [Line Items]
LIBOR basis spread on debt obligation	L+2.32 %
Rate	2.32%
Guarantee	25.00%
Advance Rate	78.75%
Margin Call	Collateral marks only
Term/Maturity	March 3, 2017

Debt Instrument Revolving Repurchase Facilities Advance Rate

No definition available.

Name:	bxmt_DebtInstrumentRevolvingRepurchaseFacilitiesAdvanceRate
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Revolving Repurchase Facilities Guarantee Rate

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentRevolvingRepurchaseFacilitiesGuaranteeRate
Namespace Prefix:	bxmt_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Revolving Repurchase Facilities Margin Call Description

+ References

No definition available.

+ Details

Name:	bxmt_DebtInstrumentRevolvingRepurchaseFacilitiesMarginCallDescription
Namespace Prefix:	bxmt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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

+ Details

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

- Details

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

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_BankOfAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_CitibankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_JpMorganMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_MetlifeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_MorganStanleyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=bxmt_WellsFargoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: