Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 03, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001061630
Current Fiscal Year End Date	--12-31
Document Period End Date	Dec. 31, 2020
Trading Symbol	BXMT
Entity Registrant Name	Blackstone Mortgage Trust, Inc.
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Public Float			$ 3.3
Entity Common Stock, Shares Outstanding		147,039,855
Document Annual Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity File Number	1-14788
Entity Tax Identification Number	94-6181186
Entity Incorporation, State or Country Code	MD
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Address, Address Line One	345 Park Avenue, 15th Floor
Entity Address, City or Town	New York
Entity Address, Postal Zip Code	10154
City Area Code	212
Local Phone Number	655-0220
Title of 12(b) Security	Class A common stock
Security Exchange Name	NYSE
Entity Address, State or Province	NY
ICFR Auditor Attestation Flag	true