Fair Values - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Significant unobservable inputs, discount rate	15.00%
Hotel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Principal Balance	$ 18.1
Number of loans - reported at fair value	2
Minimum [Member] \| Hotel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capitalization rates	9.00%
Occupancy rates	75.00%
Significant unobservable inputs, discount rate	8.00%
Maximum [Member] \| Hotel [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capitalization rates	15.00%
Occupancy rates	83.00%
Significant unobservable inputs, discount rate	12.00%

The percentage of all rental units are occupied or rented at a given time, lowest percentage in a range.

No definition available.

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Namespace Prefix:	bxmt_
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Period Type:	duration

- Definition

Indicates the number of loans under each classification reported at fair value.

+ References

No definition available.

+ Details

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- Definition

Outstanding principal amount, as of the balance sheet date, of impaired loans.

+ References

No definition available.

+ Details

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

- Details

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- Definition

Ratio of the net operating income produced by an asset and its capital cost (original prices paid to buy the asset), used as an input to measure fair value.

+ 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

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- 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

+ Details

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