Affiliates - Narrative (Details) $ in Thousands					9 Months Ended		12 Months Ended
	Apr. 06, 2020 USD ($)	Nov. 22, 2019 USD ($) shares	May 29, 2018 USD ($) warrant shares	Jan. 26, 2018 USD ($) warrant shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) loanPool	Dec. 31, 2018 transaction	Jun. 30, 2021	Dec. 31, 2020	Mar. 14, 2016
Schedule of Equity Method Investments [Line Items]
Proceeds from issuance of preferred stock and warrants, net of offering costs	$ 125,000				$ 0	$ 124,976
Investment in AS Ajax E II LLC
Schedule of Equity Method Investments [Line Items]
Percentage of additional ownership in joint venture obtained					53.10%					53.10%
2017-D
Schedule of Equity Method Investments [Line Items]
Percentage of additional ownership in joint venture obtained					50.00%
2018-C
Schedule of Equity Method Investments [Line Items]
Percentage of additional ownership in joint venture obtained					37.00%			37.00%	37.00%
Investment in Loan pool LLCs
Schedule of Equity Method Investments [Line Items]
Ownership percentage							40.40%
Number of entities | loanPool							3
Cash payment in business acquisition							$ 1,000
Great Ajax FS LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage					8.00%
Cash payment in business acquisition			$ 700	$ 1,100
Number of transactions | transaction								2
Percentage of equity interest at closing date			3.10%	4.90%
Number of warrants | warrant			3	3
Percentage of warrants exercisable			1.55%	2.45%
Number of shares (in shares) | shares			29,063	45,938
Common stock value			$ 400	$ 600
Third party | Investment in AS Ajax E II LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage by third parties					46.90%
Third party | Great Ajax II REIT
Schedule of Equity Method Investments [Line Items]
Ownership percentage by third parties					0.10%					0.10%
Third party | 2018-C
Schedule of Equity Method Investments [Line Items]
Ownership percentage by third parties					0.00%
Great Ajax Corp | Investment in AS Ajax E II LLC
Schedule of Equity Method Investments [Line Items]
Percentage of additional ownership in joint venture obtained					53.10%					53.10%
Great Ajax Corp | 2017-D
Schedule of Equity Method Investments [Line Items]
Ownership percentage by parent					50.00%					50.00%
Great Ajax Corp | Great Ajax II REIT
Schedule of Equity Method Investments [Line Items]
Ownership percentage by parent					99.90%					99.90%
Great Ajax Corp | 2018-C
Schedule of Equity Method Investments [Line Items]
Ownership percentage by parent					100.00%					63.00%
Gaea Real Estate Corp.
Schedule of Equity Method Investments [Line Items]
Proceeds from issuance of preferred stock and warrants, net of offering costs		$ 66,300
Private placement share issuance (in shares) | shares		4,419,641
Ownership percentage		23.20%			22.80%
Gaea Real Estate Corp. | Third party
Schedule of Equity Method Investments [Line Items]
Ownership percentage by third parties		76.80%			77.20%
Ajax E Master Trust | AS Ajax E LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage					5.00%
AS Ajax E LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage					16.50%					16.50%	24.20%
Thetis Asset Management LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage					19.80%
Unconsolidated Affiliates
Schedule of Equity Method Investments [Line Items]
Ownership percentage					100.00%