| Document and Entity Information | Oct. 29, 2020 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001061630 |
| Document Type | 8-K |
| Document Period End Date | Oct. 29, 2020 |
| Entity Registrant Name | Blackstone Mortgage Trust, Inc. |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 1-14788 |
| Entity Tax Identification Number | 94-6181186 |
| Entity Address, Address Line One | 345 Park Avenue |
| Entity Address, Address Line Two | 15th 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A common stock, par value $0.01 per share |
| Trading Symbol | BXMT |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Former Address [Member] | |
| Document And Entity Information [Line Items] | |
| Entity Address, Address Line One | 345 Park Avenue |
| Entity Address, Address Line Two | 42nd Floor |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10154 |
