Equity - Summary of Class A Common Stock Issuances (Parenthetical) (Detail) - $ / shares	8 Months Ended	9 Months Ended
Aug. 31, 2018	Sep. 30, 2018
Equity [Abstract]
Share issue price		$ 32.73
Net proceeds after sales discounts and commissions	$ 32.24	$ 32.26

Net proceeds after sales discounts and commissions.

No definition available.

Name:	bxmt_NetProceedsAfterSalesDiscountsAndCommissions
Namespace Prefix:	bxmt_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration