Mortgage Loans - Schedule of loan basis by year of origination (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	$ 976,351	$ 1,119,372
Mortgage loans held-for-sale, net	30,963	0
Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	539,596	424,997
California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	62,934	88,788
2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		173,221
2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		133,896
7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	143,727	142,235
4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	198,644	126,814
Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	31,450	29,421
Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	30,963
2021
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	13,686
2021 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0
2021 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0
2021 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0
2021 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0
2021 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	13,686
2021 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	0
2020
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	10,492	24,480
2020 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	762	0
2020 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0	2,221
2020 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2020 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2020 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0	0
2020 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	109	872
2020 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	9,621	21,387
2020 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	0
2019
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	9,760	7,998
2019 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	180	0
2019 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	1,265	952
2019 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2019 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2019 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	933	911
2019 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	2,802	1,397
2019 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	4,580	4,738
2019 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	332
2018
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	3,718	4,293
2018 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	697	257
2018 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	632	1,484
2018 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2018 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2018 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	108	434
2018 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	2,203	2,054
2018 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	78	64
2018 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	0
2017
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	3,935	3,800
2017 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	329	488
2017 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	370	362
2017 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2017 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		121
2017 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	435	0
2017 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	209	336
2017 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	2,592	2,493
2017 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	0
2016
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	2,688	4,520
2016 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	1,749	1,991
2016 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	0	0
2016 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2016 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		0
2016 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	449	2,125
2016 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	366	305
2016 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	124	99
2016 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	0
2009-2015
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	91,802	99,504
2009-2015 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	47,403	41,746
2009-2015 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	4,019	5,292
2009-2015 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		14,100
2009-2015 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		6,826
2009-2015 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	14,274	17,520
2009-2015 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	25,373	13,409
2009-2015 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	733	611
2009-2015 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	4,602
2006-2008
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	615,446	721,689
2006-2008 | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	357,311	280,606
2006-2008 | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	44,297	60,393
2006-2008 | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		119,343
2006-2008 | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		94,711
2006-2008 | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	95,403	88,414
2006-2008 | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	118,411	78,202
2006-2008 | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	24	20
2006-2008 | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	16,545
2005 and prior
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	224,824	253,088
2005 and prior | Great Ajax II REIT
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	131,165	99,909
2005 and prior | California
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	12,351	18,084
2005 and prior | 2018-C
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		39,778
2005 and prior | 2017-D
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis		32,238
2005 and prior | 7f7 and better
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	32,125	32,831
2005 and prior | 4f4-6f6 and below
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	49,171	30,239
2005 and prior | Non-PCD
Mortgage Loans [Line Items]
Mortgage loan portfolio by basis	12	$ 9
2005 and prior | Ajax N Trust
Mortgage Loans [Line Items]
Mortgage loans held-for-sale, net	$ 9,484