Debt - Schedule of Status of Notes and Securitizations (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Carrying value of mortgages	[1],[2]	$ 976,351	$ 1,119,372
Secured Borrowings
Debt Instrument [Line Items]
Deferred issuance costs		(8,300)	(5,400)
Mortgage loans
Debt Instrument [Line Items]
Carrying value of mortgages		790,899	842,177
Bond principal balance		$ 620,922	$ 590,761
Percentage of collateral coverage		127.00%	143.00%
Original balances at securitization cutoff date Mortgage UPB		$ 1,606,969
Original balances at securitization cutoff date Bond principal balance		1,162,564
Mortgage loans | 2017-B
Debt Instrument [Line Items]
Carrying value of mortgages		0	$ 110,062
Bond principal balance		$ 0	$ 68,729
Percentage of collateral coverage		0.00%	160.00%
Original balances at securitization cutoff date Mortgage UPB		$ 165,850
Original balances at securitization cutoff date Bond principal balance		115,846
Mortgage loans | 2017-D
Debt Instrument [Line Items]
Carrying value of mortgages		0	$ 133,897
Bond principal balance		$ 0	$ 51,256
Percentage of collateral coverage		0.00%	261.00%
Original balances at securitization cutoff date Mortgage UPB		$ 203,870
Original balances at securitization cutoff date Bond principal balance		88,903
Cash collateral for borrowed securities		26,700
Mortgage loans | 2017-D | Class A Notes
Debt Instrument [Line Items]
Original Principal			$ 102,600
Secured borrowings			51,300
Mortgage loans | 2018-C
Debt Instrument [Line Items]
Carrying value of mortgages		0	173,221
Bond principal balance		$ 0	$ 131,983
Percentage of collateral coverage		0.00%	131.00%
Original balances at securitization cutoff date Mortgage UPB		$ 222,181
Original balances at securitization cutoff date Bond principal balance		167,910
Cash collateral for borrowed securities		45,500
Mortgage loans | 2018-C | Class A Notes
Debt Instrument [Line Items]
Original Principal			$ 132,700
Secured borrowings			126,100
Mortgage loans | 2018-C | Class B Notes
Debt Instrument [Line Items]
Original Principal			15,900
Secured borrowings			5,900
Mortgage loans | 2019-D
Debt Instrument [Line Items]
Carrying value of mortgages		126,893	148,641
Bond principal balance		$ 102,367	$ 125,008
Percentage of collateral coverage		124.00%	119.00%
Original balances at securitization cutoff date Mortgage UPB		$ 193,301
Original balances at securitization cutoff date Bond principal balance		156,670
Mortgage loans | 2019-F
Debt Instrument [Line Items]
Carrying value of mortgages		121,110	$ 139,996
Bond principal balance		$ 86,702	$ 108,184
Percentage of collateral coverage		140.00%	129.00%
Original balances at securitization cutoff date Mortgage UPB		$ 170,876
Original balances at securitization cutoff date Bond principal balance		127,673
Mortgage loans | 2020-B
Debt Instrument [Line Items]
Carrying value of mortgages		123,544	$ 136,360
Bond principal balance		$ 91,203	$ 105,601
Percentage of collateral coverage		135.00%	129.00%
Original balances at securitization cutoff date Mortgage UPB		$ 156,468
Original balances at securitization cutoff date Bond principal balance		114,534
Mortgage loans | 2021-A
Debt Instrument [Line Items]
Carrying value of mortgages		168,049	$ 0
Bond principal balance		$ 149,730	$ 0
Percentage of collateral coverage		112.00%	0.00%
Original balances at securitization cutoff date Mortgage UPB		$ 206,528
Original balances at securitization cutoff date Bond principal balance		175,116
Mortgage loans | 2021-B
Debt Instrument [Line Items]
Carrying value of mortgages		251,303	$ 0
Bond principal balance		$ 190,920	$ 0
Percentage of collateral coverage		132.00%	0.00%
Original balances at securitization cutoff date Mortgage UPB		$ 287,895
Original balances at securitization cutoff date Bond principal balance		$ 215,912

[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.