Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity Schedule of Common Dividends Declared (Details)

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Stockholders' Equity Schedule of Common Dividends Declared (Details) - $ / shares
1 Months Ended 2 Months Ended 3 Months Ended 9 Months Ended
Nov. 01, 2017
Jul. 31, 2018
Sep. 30, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Class of Stock [Line Items]                        
Dividends declared per common share (in usd per share)       $ 0.47       $ 0.52     $ 1.41 $ 1.54
Number of shares of common stock of Granite Point Mortgage Trust Inc distributed via special dividend (in shares) 33,100,000                      
Common Stock [Member]                        
Class of Stock [Line Items]                        
Declaration Date   Jul. 13, 2018 Sep. 20, 2018   Jun. 19, 2018 Mar. 20, 2018 Dec. 14, 2017 Sep. 14, 2017 Jun. 15, 2017 Mar. 14, 2017    
Record Date   Jul. 25, 2018 Oct. 01, 2018   Jun. 29, 2018 Apr. 02, 2018 Dec. 26, 2017 Sep. 29, 2017 Jun. 30, 2017 Mar. 31, 2017    
Payment Date   Jul. 30, 2018 Oct. 29, 2018   Jul. 27, 2018 Apr. 27, 2018 Dec. 29, 2017 Oct. 27, 2017 Jul. 27, 2017 Apr. 27, 2017    
Dividends declared per common share (in usd per share)   $ 0.158370 $ 0.311630   $ 0.470000 $ 0.470000 $ 0.470000 $ 0.520000 $ 0.520000 $ 0.500000