Basis of Presentation and Significant Accounting Policies Consolidation and Basis of Presentation (Details) - Subsequent Event [Member]	Nov. 01, 2017 shares
Subsequent Event [Line Items]
Reverse stock split, conversion ratio	0.50
Number of shares of common stock of Granite Point Mortgage Trust Inc distributed via special dividend (in shares)	33,100,000

Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB TOPIC 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Stock Split
-URI http://asc.fasb.org/extlink&oid=6525746

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