Fair Value - Schedule of Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Financial assets
Mortgage loans held-for-investment, net	[1],[2]	$ 976,351	$ 1,119,372
Mortgage loans held-for-sale, net		30,963	0
Financial liabilities
Borrowings under repurchase transactions		399,340	421,132
Convertible senior notes, net	[3]	103,754	110,057
Level 1 - Quoted prices in active markets
Financial assets
Mortgage loans, net fair value		0	0
Mortgage loans held-for-sale, net		0
Financial liabilities
Secured borrowings, fair value		0	0
Borrowings under repurchase agreement, fair value		0	0
Convertible senior notes, net, fair value		111,227	110,675
Level 1 - Quoted prices in active markets | Investment in Manager
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Investment in AS Ajax E LLC
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Investment in AS Ajax E II LLC
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Great Ajax F S
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Gaea Real Estate Corp.
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Investment in Loan pool LLCs
Financial assets
Investments in affiliates, fair value		0	0
Level 1 - Quoted prices in active markets | Beneficial interests in securitization trusts
Financial assets
Investments, fair value disclosure		0	0
Level 2 - Observable inputs other than Level 1 prices
Financial assets
Mortgage loans, net fair value		0	0
Mortgage loans held-for-sale, net		0
Financial liabilities
Secured borrowings, fair value		624,743	586,419
Borrowings under repurchase agreement, fair value		399,340	421,132
Convertible senior notes, net, fair value		0	0
Level 2 - Observable inputs other than Level 1 prices | Investment in Manager
Financial assets
Investments in affiliates, fair value		0	0
Level 2 - Observable inputs other than Level 1 prices | Investment in AS Ajax E LLC
Financial assets
Investments in affiliates, fair value		761	934
Level 2 - Observable inputs other than Level 1 prices | Investment in AS Ajax E II LLC
Financial assets
Investments in affiliates, fair value		2,958	3,484
Level 2 - Observable inputs other than Level 1 prices | Great Ajax F S
Financial assets
Investments in affiliates, fair value		0	0
Level 2 - Observable inputs other than Level 1 prices | Gaea Real Estate Corp.
Financial assets
Investments in affiliates, fair value		0	0
Level 2 - Observable inputs other than Level 1 prices | Investment in Loan pool LLCs
Financial assets
Investments in affiliates, fair value		0	0
Level 2 - Observable inputs other than Level 1 prices | Beneficial interests in securitization trusts
Financial assets
Investments, fair value disclosure		0	0
Level 3 - Unobservable inputs
Financial assets
Mortgage loans, net fair value		1,059,040	1,232,081
Mortgage loans held-for-sale, net		33,429
Financial liabilities
Secured borrowings, fair value		0	0
Borrowings under repurchase agreement, fair value		0	0
Convertible senior notes, net, fair value		0	0
Level 3 - Unobservable inputs | Investment in Manager
Financial assets
Investments in affiliates, fair value		12,379	11,709
Level 3 - Unobservable inputs | Investment in AS Ajax E LLC
Financial assets
Investments in affiliates, fair value		0	0
Level 3 - Unobservable inputs | Investment in AS Ajax E II LLC
Financial assets
Investments in affiliates, fair value		0	0
Level 3 - Unobservable inputs | Great Ajax F S
Financial assets
Investments in affiliates, fair value		3,320	3,320
Level 3 - Unobservable inputs | Gaea Real Estate Corp.
Financial assets
Investments in affiliates, fair value		20,287	19,150
Level 3 - Unobservable inputs | Investment in Loan pool LLCs
Financial assets
Investments in affiliates, fair value		672	701
Level 3 - Unobservable inputs | Beneficial interests in securitization trusts
Financial assets
Investments, fair value disclosure		139,494	91,418
Carrying value
Financial assets
Mortgage loans held-for-investment, net		976,351	1,119,372
Mortgage loans held-for-sale, net		30,963
Financial liabilities
Secured borrowings, net		612,592	585,403
Borrowings under repurchase transactions		399,340	421,132
Convertible senior notes, net		103,754	110,057
Carrying value | Investment in Manager
Financial assets
Investments In affiliates		1,569	1,366
Carrying value | Investment in AS Ajax E LLC
Financial assets
Investments In affiliates		611	776
Carrying value | Investment in AS Ajax E II LLC
Financial assets
Investments In affiliates		2,755	3,381
Carrying value | Great Ajax F S
Financial assets
Investments In affiliates		2,619	2,711
Carrying value | Gaea Real Estate Corp.
Financial assets
Investments In affiliates		19,670	20,001
Carrying value | Investment in Loan pool LLCs
Financial assets
Investments In affiliates		240	381
Carrying value | Beneficial interests in securitization trusts
Financial assets
Investments In affiliates		$ 139,494	$ 91,418

[1] As of September 30, 2021, balances for Mortgage loans held-for-investment, net include $1.5 million from a 50.0% owned joint venture. As of December 31, 2020, balances for Mortgage loans held-for-investment, net include $307.1 million and Secured borrowings, net of deferred costs includes $250.6 million from 50.0% and 63.0% owned joint ventures, all of which the Company consolidates under U.S. Generally Accepted Accounting Principles ("U.S. GAAP"). The creditors do not have recourse to the primary beneficiary (Great Ajax Corp.). See Note 9 — Debt.    

[2] Mortgage loans held-for-investment, net include $790.9 million and $842.2 million of loans at September 30, 2021 and December 31, 2020, respectively, transferred to securitization trusts that are variable interest entities (“VIEs”); these loans can only be used to settle obligations of the VIEs. Secured borrowings consist of notes issued by VIEs that can only be settled with the assets and cash flows of the VIEs. The creditors do not have recourse to the primary beneficiary (Great Ajax Corp.). See Note 9 — Debt. Mortgage loans held-for-investment, net include $13.9 million and $13.7 million of allowance for expected credit losses at September 30, 2021 and December 31, 2020, respectively.

[3] Secured borrowings, net are presented net of deferred issuance costs of $8.3 million at September 30, 2021 and $5.4 million at December 31, 2020. Convertible senior notes, net are presented net of deferred issuance costs of $2.1 million at September 30, 2021 and $3.3 million at December 31, 2020.