Fair Values - Schedule of Range of Key Assumptions for Each Type of Loans Receivable (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount Rate	15.00%
Book Value	$ 66,063
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Book Value	26,500
Book Value Sensitivity to 100 bp Discount Rate Increase	(0.70%)
Hotel [Member] \| Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount Rate	8.00%
Recovery Percentage	84.00%
Hotel [Member] \| Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount Rate	9.00%
Recovery Percentage	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Book Value	$ 39,600
Book Value Sensitivity to 100 bp Discount Rate Increase	(1.50%)
Office [Member] \| Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount Rate	7.00%
Recovery Percentage	78.00%
Office [Member] \| Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount Rate	20.00%
Recovery Percentage	100.00%

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

Percentage of likelihood a loan will be repaid relative to the loan balance, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
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=25499696&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 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