Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans \| SecurityLoan	116	125
Net book value	$ 9,090,934	$ 9,077,007
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans \| SecurityLoan	116	125
Principal balance	$ 9,122,567	$ 9,108,361
Unfunded loan commitments	$ 907,709	$ 700,658
Weighted-average maximum maturity (years)	3 years	3 years 1 month 6 days
Loans Receivable [Member] \| LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.85%	4.84%
Weighted-average all-in yield, rate	5.19%	5.18%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,090,934	$ 9,077,007

Unfunded Commitments Related To Loans Receivable

No definition available.

Name:	bxmt_UnfundedCommitmentsRelatedToLoansReceivable
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted Average Cash Coupon Rate

+ References

No definition available.

+ Details

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

- Definition

Weighted Average Maturity Term

+ References

No definition available.

+ Details

Name:	bxmt_WeightedAverageMaturityTerm
Namespace Prefix:	bxmt_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted Average Yield Cost Rate

+ References

No definition available.

+ Details

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

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5074-111524

+ Details

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

- Definition

Indicates the number of mortgages under each classification.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29.3)
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 10
-Section S99
-Paragraph 6
-Subparagraph (SX 210.5-04.(c) Schedule IV)
-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateNumberOfLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=bxmt_BookValueNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: