FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Assets:
Fair value of assets		$ 3,564,681	$ 3,615,097
Provision for loan losses	[1]	(17,900)	(4,000)
Liabilities:
Fair value of liabilities		4,401,295	4,414,782
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount		436,957	371,427
Fair value of liabilities		436,957	371,427
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount		226,728	101,983
Fair value of liabilities		226,728	101,983
Mortgage loan financing
Liabilities:
Outstanding Face Amount		738,825	692,394
Fair value of liabilities		735,662	693,055
CLO debt
Liabilities:
Outstanding Face Amount		601,543
Fair value of liabilities		601,543
Participation Financing - Mortgage Loan Receivable
Liabilities:
Outstanding Face Amount		2,453	688,479
Fair value of liabilities		2,453	688,479
Senior unsecured notes
Liabilities:
Outstanding Face Amount		1,166,201	1,166,201
Fair value of liabilities		1,111,288	1,187,187
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount		1,286,000	1,370,000
Fair value of liabilities		1,286,664	1,369,544
Liability For Transfers Not Considered Sales
Liabilities:
Outstanding Face Amount			3,107
Fair value of liabilities			3,107
CMBS
Assets:
Outstanding Face Amount		12,210	12,293
Fair value of assets		11,306	11,651
CMBS interest-only
Assets:
Outstanding Face Amount		11,189	11,271
Fair value of assets		902	1,068
Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Outstanding Face Amount		3,340,381	3,300,709
Fair value of assets		3,324,588	3,292,035
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount		181,905	232,527
Fair value of assets		187,870	236,428
FHLB stock
Assets:
Outstanding Face Amount		57,915	77,915
Fair value of assets		57,915	77,915
Level 1
Assets:
Fair value of assets		0	0
Provision for loan losses		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 | Mortgage loan financing
Liabilities:
Fair value of liabilities		0	0
Level 1 | CLO debt
Liabilities:
Fair value of liabilities		0
Level 1 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair value of liabilities		0	0
Level 1 | Senior unsecured notes
Liabilities:
Fair value of liabilities		0	0
Level 1 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities		0	0
Level 1 | Liability For Transfers Not Considered Sales
Liabilities:
Fair value of liabilities			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 | 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	0
Level 1 | FHLB stock
Assets:
Fair value of assets		0	0
Level 2
Assets:
Fair value of assets		0	0
Provision for loan losses		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 | Mortgage loan financing
Liabilities:
Fair value of liabilities		0	0
Level 2 | CLO debt
Liabilities:
Fair value of liabilities		0
Level 2 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair value of liabilities		0	0
Level 2 | Senior unsecured notes
Liabilities:
Fair value of liabilities		0	0
Level 2 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities		0	0
Level 2 | Liability For Transfers Not Considered Sales
Liabilities:
Fair value of liabilities			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 | 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	0
Level 2 | FHLB stock
Assets:
Fair value of assets		0	0
Level 3
Assets:
Fair value of assets		3,564,681	3,615,097
Provision for loan losses		(17,900)	(4,000)
Liabilities:
Fair value of liabilities		4,401,295	4,414,782
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair value of liabilities		436,957	371,427
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair value of liabilities		226,728	101,983
Level 3 | Mortgage loan financing
Liabilities:
Fair value of liabilities		735,662	693,055
Level 3 | CLO debt
Liabilities:
Fair value of liabilities		601,543
Level 3 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair value of liabilities		2,453	688,479
Level 3 | Senior unsecured notes
Liabilities:
Fair value of liabilities		1,111,288	1,187,187
Level 3 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities		1,286,664	1,369,544
Level 3 | Liability For Transfers Not Considered Sales
Liabilities:
Fair value of liabilities			3,107
Level 3 | CMBS
Assets:
Fair value of assets		11,306	11,651
Level 3 | CMBS interest-only
Assets:
Fair value of assets		902	1,068
Level 3 | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair value of assets		3,324,588	3,292,035
Level 3 | Mortgage loan receivables held for sale
Assets:
Fair value of assets		187,870	236,428
Level 3 | FHLB stock
Assets:
Fair value of assets		57,915	77,915
Recurring
Assets:
Fair value of assets		1,397,918	1,094,687
Recurring | Nonhedge derivatives
Assets:
Fair value of assets		975
Liabilities:
Nonhedge derivative liabilities		605,871	54,160
Fair value of liabilities			2,606
Recurring | CMBS
Assets:
Outstanding Face Amount		1,246,609	932,874
Fair value of assets		1,241,334	941,849
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount		2,362,747	3,129,027
Fair value of assets		54,789	112,003
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount		135,932	172,916
Fair value of assets		2,648	4,477
Recurring | Agency securities
Assets:
Outstanding Face Amount		668	720
Fair value of assets		662	728
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount		32,633	33,745
Fair value of assets		33,064	34,742
Recurring | Corporate bonds
Assets:
Outstanding Face Amount		55,305
Fair value of assets		53,871
Recurring | Nonhedge derivatives
Assets:
Fair value of assets			888
Derivative Asset, Notional Amount			594,140
Recurring | Common Stock
Assets:
Fair value of assets		11,550
Recurring | Level 1
Assets:
Fair value of assets		11,550	0
Recurring | Level 1 | Nonhedge derivatives
Assets:
Fair value of assets		0
Liabilities:
Fair value of liabilities			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	0
Recurring | Level 1 | Corporate bonds
Assets:
Fair value of assets		0
Recurring | Level 1 | Nonhedge derivatives
Assets:
Fair value of assets			0
Recurring | Level 1 | Common Stock
Assets:
Fair value of assets		11,550
Recurring | Level 2
Assets:
Fair value of assets		0	888
Recurring | Level 2 | Nonhedge derivatives
Liabilities:
Fair value of liabilities		975	2,606
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	0
Recurring | Level 2 | Corporate bonds
Assets:
Fair value of assets		0
Recurring | Level 2 | Nonhedge derivatives
Assets:
Fair value of assets			888
Recurring | Level 3
Assets:
Fair value of assets		1,386,368	1,093,799
Recurring | Level 3 | Nonhedge derivatives
Liabilities:
Fair value of liabilities		0	0
Recurring | Level 3 | CMBS
Assets:
Fair value of assets		1,241,334	941,849
Recurring | Level 3 | CMBS interest-only
Assets:
Fair value of assets		54,789	112,003
Recurring | Level 3 | GNMA interest-only
Assets:
Fair value of assets		2,648	4,477
Recurring | Level 3 | Agency securities
Assets:
Fair value of assets		662	728
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair value of assets		33,064	34,742
Recurring | Level 3 | Corporate bonds
Assets:
Fair value of assets		$ 53,871
Recurring | Level 3 | Nonhedge derivatives
Assets:
Fair value of assets			$ 0

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.