Fair Values - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015 SecurityLoan	Dec. 31, 2014 SecurityLoan
Fair Value Disclosures [Abstract]
Book value of securities	$ 9,900,000
Loans receivable	4,000,000	19,000,000
Number of loans	1bxmt_LoansReceivableNumberOfLoans	2bxmt_LoansReceivableNumberOfLoans
Discount rate	15.00%us-gaap_FairValueInputsDiscountRate	15.00%us-gaap_FairValueInputsDiscountRate
Discount rate on loans repaid during 2015		7.00%
Fair value of a 100 bp discount rate increase	0.90%	0.50%
Aggregate book value of equity investments and other receivables	36,100,000
Liabilities recorded at fair value	$ 0us-gaap_LiabilitiesFairValueDisclosure	$ 0us-gaap_LiabilitiesFairValueDisclosure

The percent of book value sensitivity to a 100 bps discount rate increase used as an input to measure fair value.

No definition available.

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

- Definition

Equity Investments and Other Receivables Carrying Value

+ References

No definition available.

+ Details

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

- Definition

Fair value inputs, discount rate on loans repaid in next fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Loans Receivable Number Of Loans

+ References

No definition available.

+ Details

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

- Definition

Investments in notes or bonds that are collateralized by a specific group of underlying assets owned by the issuer of the notes or bonds which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

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

- Details

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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

- Definition

Fair value of financial and nonfinancial obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

+ Details

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