Basis of Presentation and Significant Accounting Policies Consolidation and Basis of Presentation (Details) shares in Millions	12 Months Ended
Dec. 31, 2017 shares
Basis of Presentation and Significant Accounting Policies [Abstract]
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)	33.1

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://fasb.org/us-gaap/role/ref/legacyRef
-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

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Period Type:	duration