Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Receivables [Abstract]
Total loan exposure including senior interests	$ 987.6	$ 1,000.0

Loan exposure amount including non consolidated senior interests.

No definition available.

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