Equity - Schedule of Movement in Outstanding Shares of Class A Common Stock, Restricted Class A Common Stock and Deferred Stock Units (Parenthetical) (Detail)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred stock units held by directors	118,919bxmt_DeferredStockUnits	101,233bxmt_DeferredStockUnits	89,754bxmt_DeferredStockUnits
Class A Common Stock [Member]
Reverse stock split ratio	0.1

Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Stock Split

-URI http://asc.fasb.org/extlink&oid=6525746

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 4

-Subparagraph (SAB TOPIC 4.C)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

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