Document - shares	6 Months Ended
	Jun. 30, 2019	Aug. 02, 2019
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2019
Document Transition Report	false
Entity File Number	001-38124
Entity Registrant Name	GRANITE POINT MORTGAGE TRUST INC.
Entity Central Index Key	0001703644
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	61-1843143
Entity Address, Address Line One	3 Bryant Park, 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
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
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		54,853,205
NEW YORK STOCK EXCHANGE, INC. [Member]
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	GPMT
Security Exchange Name	NYSE