Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity Schedule of Common Dividends Declared (Details)

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Stockholders' Equity Schedule of Common Dividends Declared (Details) - $ / shares
3 Months Ended
Nov. 01, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Class of Stock [Line Items]                          
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   Dec. 14, 2017 Sep. 14, 2017 Jun. 15, 2017 Mar. 14, 2017 Dec. 15, 2016 Sep. 15, 2016 Jun. 16, 2016 Mar. 15, 2016 Dec. 16, 2015 Sep. 16, 2015 Jun. 17, 2015 Mar. 18, 2015
Record Date   Dec. 26, 2017 Sep. 29, 2017 Jun. 30, 2017 Mar. 31, 2017 Dec. 30, 2016 Sep. 30, 2016 Jun. 30, 2016 Mar. 31, 2016 Dec. 30, 2015 Sep. 30, 2015 Jun. 30, 2015 Mar. 31, 2015
Payment Date   Dec. 29, 2017 Oct. 27, 2017 Jul. 27, 2017 Apr. 27, 2017 Jan. 27, 2017 Oct. 20, 2016 Jul. 20, 2016 Apr. 21, 2016 Jan. 20, 2016 Oct. 22, 2015 Jul. 21, 2015 Apr. 21, 2015
Dividends declared per common share (in usd per share)   $ 0.47 $ 0.52 $ 0.52 $ 0.5 $ 0.48 $ 0.46 $ 0.46 $ 0.46 $ 0.52 $ 0.52 $ 0.52 $ 0.52