| Cover | Oct. 10, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 10, 2020 |
| Entity File Number | 001-38124 |
| Entity Registrant Name | Granite Point Mortgage Trust Inc. |
| Entity Central Index Key | 0001703644 |
| Entity Tax Identification Number | 61-1843143 |
| Entity Incorporation, State or Country Code | MD |
| Entity Address, Address Line One | 3 Bryant Park |
| Entity Address, Address Line Two | Suite 2400A |
| Entity Address, City or Town | New York |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 10036 |
| City Area Code | 212 |
| Local Phone Number | 364-3200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | GPMT |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
