Commitments and Contingencies - Schedule of Principal Debt Repayments (Parenthetical) (Detail) $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Non-consolidated senior interests excluded from contractual obligations	$ 406.3
Securitized debt obligations excluded from contractual obligations	$ 1,300.0
Debt Instrument Amortization Percentage	1.00%

Non-consolidated senior interests excluded from contractual obligations.

No definition available.

Name:	bxmt_NonconsolidatedSeniorInterestsExcludedFromContractualObligations
Namespace Prefix:	bxmt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant