Stockholders' Equity - Schedule of Primary Components of Accumulated Other Comprehensive Loss (Parenthetical) (Detail) (USD $)	6 Months Ended
Jun. 30, 2013
Accumulated Other Comprehensive Loss [Abstract]
Other-than-temporary impairments of securities	$ 391,000

Other Than Temporary Impairment Losses Investments Portion Recognized In Securities

No definition available.

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