Equity - Summary of Class A Common Stock Issuances (Parenthetical) (Detail)	3 Months Ended
Jun. 30, 2018 $ / shares
Equity [Abstract]
Net proceeds after sales discounts and commissions	$ 31.57

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