Organization and Basis of Presentation (Details)			9 Months Ended
	Apr. 06, 2020 USD ($)	Nov. 22, 2019 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) payment entity	Sep. 30, 2020 USD ($)	Jun. 30, 2021	Dec. 31, 2020 entity	Dec. 31, 2018
Organization And Basis Of Presentation [Line Items]
Number of payments made on RPL mortgage loans (at least) | payment			5
Number of recent payments made on RPL mortgage loans | payment			7
Number of payments made on NPL mortgage loans | payment			3
Proceeds from issuance of preferred stock and warrants, net of offering costs | $	$ 125,000,000		$ 0	$ 124,976,000
Number of non controlling interest subsidiaries | entity			3			4
Delaware Trust
Organization And Basis Of Presentation [Line Items]
Ownership percentage			5.00%
Investment in AS Ajax E II LLC
Organization And Basis Of Presentation [Line Items]
Percentage of additional ownership in joint venture obtained			53.10%			53.10%
Investment in AS Ajax E II LLC | Great Ajax Corp
Organization And Basis Of Presentation [Line Items]
Percentage of additional ownership in joint venture obtained			53.10%			53.10%
Investment in AS Ajax E II LLC | Third party
Organization And Basis Of Presentation [Line Items]
Ownership percentage by third parties			46.90%
2017-D
Organization And Basis Of Presentation [Line Items]
Percentage of additional ownership in joint venture obtained			50.00%
2017-D | Great Ajax Corp
Organization And Basis Of Presentation [Line Items]
Ownership percentage by parent			50.00%			50.00%
Great Ajax II REIT | Great Ajax Corp
Organization And Basis Of Presentation [Line Items]
Ownership percentage by parent			99.90%			99.90%
Great Ajax II REIT | Third party
Organization And Basis Of Presentation [Line Items]
Ownership percentage by third parties			0.10%			0.10%
2018-C
Organization And Basis Of Presentation [Line Items]
Percentage of additional ownership in joint venture obtained			37.00%		37.00%		37.00%
2018-C | Great Ajax Corp
Organization And Basis Of Presentation [Line Items]
Ownership percentage by parent			100.00%			63.00%
2018-C | Third party
Organization And Basis Of Presentation [Line Items]
Ownership percentage by third parties			0.00%
Servicer
Organization And Basis Of Presentation [Line Items]
Ownership percentage			8.00%
Thetis Asset Management LLC
Organization And Basis Of Presentation [Line Items]
Ownership percentage			19.80%
Gaea Real Estate Corp.
Organization And Basis Of Presentation [Line Items]
Ownership percentage		23.20%	22.80%
Proceeds from issuance of preferred stock and warrants, net of offering costs | $		$ 66,300,000
Private placement share issuance (in shares) | shares		4,419,641
Purchase price per share (in usd per share) | $ / shares		$ 15.00
Gaea Real Estate Corp. | Third party
Organization And Basis Of Presentation [Line Items]
Ownership percentage by third parties		76.80%	77.20%
Maximum
Organization And Basis Of Presentation [Line Items]
Principal balance of small balance commercial mortgage loans (up to) | $			$ 5,000,000