Cover Page - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2021	Feb. 02, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-14788
Entity Registrant Name	Blackstone Mortgage Trust, Inc.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	94-6181186
Entity Address, Address Line One	345 Park Avenue
Entity Address, Address Line Two	24th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10154
City Area Code	212
Local Phone Number	655-0220
Title of 12(b) Security	Class A common stock,
Trading Symbol	BXMT
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4.5
Entity Common Stock, Shares Outstanding		168,608,071
Documents Incorporated by Reference	Part III of this annual report on Form 10-K incorporates information by reference from the registrant’s definitive proxy statement with respect to its 2021 annual meeting of stockholders to be filed with the Securities and Exchange Commission within 120 days after the close of the registrant’s fiscal year.
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Central Index Key	0001061630