FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020
Assets:
Fair value of assets	$ 3,516,524	$ 2,403,272
Allowance for current expected credit losses	(31,752)	(41,507)	$ (11,600)
Liabilities:
Fair value of liabilities	4,282,888	4,247,552
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	418,394	708,833
Fair value of liabilities	418,394	708,833
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	26,183	112,004
Fair value of liabilities	26,183	112,004
Revolving credit facility
Liabilities:
Outstanding Face Amount		266,430
Fair value of liabilities		266,430
Mortgage loan financing
Liabilities:
Outstanding Face Amount	690,927	761,793
Fair value of liabilities	709,695	786,405
Secured financing facility
Liabilities:
Outstanding Face Amount	136,444	206,350
Fair value of liabilities	133,389	200,343
CLO debt
Liabilities:
Outstanding Face Amount	1,064,365	276,516
Fair value of liabilities	1,054,774	276,516
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	263,000	288,000
Fair value of liabilities	263,414	289,091
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,649,794	1,612,299
Fair value of liabilities	1,677,039	1,607,930
CMBS
Assets:
Principal Amount	10,326	11,523
Fair value of assets	9,894	11,074
CMBS interest-only
Assets:
Principal Amount	9,370	10,566
Fair value of assets	541	675
Total mortgage loan receivables held for investment, net, at amortized cost
Assets:
Principal Amount	3,581,920	2,365,204
Fair value of assets	3,494,254	2,328,441
Mortgage loan  receivables held for sale
Assets:
Principal Amount		30,478
Fair value of assets		32,082
FHLB stock
Assets:
Principal Amount	11,835	31,000
Fair value of assets	11,835	31,000
Level 1
Assets:
Fair value of assets	0	0
Liabilities:
Fair value of liabilities	0	0
Level 1 | Repurchase agreements - short-term
Liabilities:
Fair value of liabilities	0	0
Level 1 | Repurchase agreements - long-term
Liabilities:
Fair value of liabilities	0	0
Level 1 | Revolving credit facility
Liabilities:
Fair value of liabilities		0
Level 1 | Mortgage loan financing
Liabilities:
Fair value of liabilities	0	0
Level 1 | Secured financing facility
Liabilities:
Fair value of liabilities	0	0
Level 1 | CLO debt
Liabilities:
Fair value of liabilities	0	0
Level 1 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities	0	0
Level 1 | Senior unsecured notes
Liabilities:
Fair value of liabilities	0	0
Level 1 | CMBS
Assets:
Fair value of assets	0	0
Level 1 | CMBS interest-only
Assets:
Fair value of assets	0	0
Level 1 | Total mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair value of assets	0	0
Level 1 | Mortgage loan  receivables held for sale
Assets:
Fair value of assets		0
Level 1 | FHLB stock
Assets:
Fair value of assets	0	0
Level 2
Assets:
Fair value of assets	0	0
Liabilities:
Fair value of liabilities	0	0
Level 2 | Repurchase agreements - short-term
Liabilities:
Fair value of liabilities	0	0
Level 2 | Repurchase agreements - long-term
Liabilities:
Fair value of liabilities	0	0
Level 2 | Revolving credit facility
Liabilities:
Fair value of liabilities		0
Level 2 | Mortgage loan financing
Liabilities:
Fair value of liabilities	0	0
Level 2 | Secured financing facility
Liabilities:
Fair value of liabilities	0	0
Level 2 | CLO debt
Liabilities:
Fair value of liabilities	0	0
Level 2 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities	0	0
Level 2 | Senior unsecured notes
Liabilities:
Fair value of liabilities	0	0
Level 2 | CMBS
Assets:
Fair value of assets	0	0
Level 2 | CMBS interest-only
Assets:
Fair value of assets	0	0
Level 2 | Total mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair value of assets	0	0
Level 2 | Mortgage loan  receivables held for sale
Assets:
Fair value of assets		0
Level 2 | FHLB stock
Assets:
Fair value of assets	0	0
Level 3
Assets:
Fair value of assets	3,516,524	2,403,272
Liabilities:
Fair value of liabilities	4,282,888	4,247,552
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair value of liabilities	418,394	708,833
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair value of liabilities	26,183	112,004
Level 3 | Revolving credit facility
Liabilities:
Fair value of liabilities		266,430
Level 3 | Mortgage loan financing
Liabilities:
Fair value of liabilities	709,695	786,405
Level 3 | Secured financing facility
Liabilities:
Fair value of liabilities	133,389	200,343
Level 3 | CLO debt
Liabilities:
Fair value of liabilities	1,054,774	276,516
Level 3 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities	263,414	289,091
Level 3 | Senior unsecured notes
Liabilities:
Fair value of liabilities	1,677,039	1,607,930
Level 3 | CMBS
Assets:
Fair value of assets	9,894	11,074
Level 3 | CMBS interest-only
Assets:
Fair value of assets	541	675
Level 3 | Total mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair value of assets	3,494,254	2,328,441
Level 3 | Mortgage loan  receivables held for sale
Assets:
Fair value of assets		32,082
Level 3 | FHLB stock
Assets:
Fair value of assets	11,835	31,000
Recurring
Assets:
Fair value of assets	693,266	1,046,869
Recurring | CMBS
Assets:
Principal Amount	681,076	1,003,998
Fair value of assets	676,398	992,227
Recurring | CMBS interest-only
Assets:
Principal Amount	1,293,181	1,487,616
Fair value of assets	15,344	21,538
Recurring | GNMA interest-only
Assets:
Principal Amount	59,075	75,350
Fair value of assets	559	1,001
Recurring | Agency securities
Assets:
Principal Amount	557	586
Fair value of assets	563	605
Recurring | GNMA permanent securities
Assets:
Principal Amount		30,254
Fair value of assets		31,199
Recurring | Nonhedge derivatives
Assets:
Fair value of assets	402	299
Nonhedge derivative assets	114,121	65,600
Recurring | Level 1
Assets:
Fair value of assets	0	0
Recurring | Level 1 | CMBS
Assets:
Fair value of assets	0	0
Recurring | Level 1 | CMBS interest-only
Assets:
Fair value of assets	0	0
Recurring | Level 1 | GNMA interest-only
Assets:
Fair value of assets	0	0
Recurring | Level 1 | Agency securities
Assets:
Fair value of assets	0	0
Recurring | Level 1 | GNMA permanent securities
Assets:
Fair value of assets		0
Recurring | Level 1 | Nonhedge derivatives
Assets:
Fair value of assets	0	0
Recurring | Level 2
Assets:
Fair value of assets	402	299
Recurring | Level 2 | CMBS
Assets:
Fair value of assets	0	0
Recurring | Level 2 | CMBS interest-only
Assets:
Fair value of assets	0	0
Recurring | Level 2 | GNMA interest-only
Assets:
Fair value of assets	0	0
Recurring | Level 2 | Agency securities
Assets:
Fair value of assets	0	0
Recurring | Level 2 | GNMA permanent securities
Assets:
Fair value of assets		0
Recurring | Level 2 | Nonhedge derivatives
Assets:
Fair value of assets	402	299
Recurring | Level 3
Assets:
Fair value of assets	692,864	1,046,570
Recurring | Level 3 | CMBS
Assets:
Fair value of assets	676,398	992,227
Recurring | Level 3 | CMBS interest-only
Assets:
Fair value of assets	15,344	21,538
Recurring | Level 3 | GNMA interest-only
Assets:
Fair value of assets	559	1,001
Recurring | Level 3 | Agency securities
Assets:
Fair value of assets	563	605
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair value of assets		31,199
Recurring | Level 3 | Nonhedge derivatives
Assets:
Fair value of assets	$ 0	$ 0