Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 20, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Trading Symbol	BXMT
Entity Registrant Name	BLACKSTONE MORTGAGE TRUST, INC.
Entity Central Index Key	0001061630
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		93,213,042