Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Total loan exposure including senior interests	$ 995.9	$ 985.4

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