FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Assets:
Fair Value	$ 3,602,378		$ 2,452,785
Provision for loan losses	(4,000)	[1]	(4,000)
Liabilities:
Fair Value	4,414,782		3,940,704
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	371,427		629,430
Fair Value	371,427		629,430
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	101,983		477,756
Fair Value	101,983		477,756
Revolving credit facility
Liabilities:
Outstanding Face Amount			25,000
Fair Value			25,000
Mortgage loan financing
Liabilities:
Outstanding Face Amount	692,394		589,152
Fair Value	693,055		595,778
CLO debt
Liabilities:
Outstanding Face Amount	688,479
Fair Value	688,479
Participation Financing - Mortgage Loan Receivable
Liabilities:
Outstanding Face Amount	3,107
Fair Value	3,107
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,370,000		1,660,000
Fair Value	1,369,544		1,662,178
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,166,201		563,872
Fair Value	1,187,187		550,562
Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Outstanding Face Amount	3,300,709		2,011,309
Fair Value	3,292,035		2,018,973
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	232,527		360,518
Fair Value	236,428		359,897
FHLB stock
Assets:
Outstanding Face Amount	77,915		77,915
Fair Value	77,915		77,915
Level 1
Assets:
Fair Value	0		0
Provision for loan losses	0		0
Liabilities:
Fair Value	0		0
Level 1 | Repurchase agreements - short-term
Liabilities:
Fair Value	0		0
Level 1 | Repurchase agreements - long-term
Liabilities:
Fair Value	0		0
Level 1 | Revolving credit facility
Liabilities:
Fair Value			0
Level 1 | Mortgage loan financing
Liabilities:
Fair Value	0		0
Level 1 | CLO debt
Liabilities:
Fair Value	0
Level 1 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair Value	0
Level 1 | Borrowings from the FHLB
Liabilities:
Fair Value	0		0
Level 1 | Senior unsecured notes
Liabilities:
Fair Value	0		0
Level 1 | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair Value	0		0
Level 1 | Mortgage loan receivables held for sale
Assets:
Fair Value	0		0
Level 1 | FHLB stock
Assets:
Fair Value	0		0
Level 2
Assets:
Fair Value	0		0
Provision for loan losses	0		0
Liabilities:
Fair Value	0		0
Level 2 | Repurchase agreements - short-term
Liabilities:
Fair Value	0		0
Level 2 | Repurchase agreements - long-term
Liabilities:
Fair Value	0		0
Level 2 | Revolving credit facility
Liabilities:
Fair Value			0
Level 2 | Mortgage loan financing
Liabilities:
Fair Value	0		0
Level 2 | CLO debt
Liabilities:
Fair Value	0
Level 2 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair Value	0
Level 2 | Borrowings from the FHLB
Liabilities:
Fair Value	0		0
Level 2 | Senior unsecured notes
Liabilities:
Fair Value	0		0
Level 2 | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair Value	0		0
Level 2 | Mortgage loan receivables held for sale
Assets:
Fair Value	0		0
Level 2 | FHLB stock
Assets:
Fair Value	0		0
Level 3
Assets:
Fair Value	3,602,378		2,452,785
Provision for loan losses	(4,000)		(4,000)
Liabilities:
Fair Value	4,414,782		3,940,704
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair Value	371,427		629,430
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair Value	101,983		477,756
Level 3 | Revolving credit facility
Liabilities:
Fair Value			25,000
Level 3 | Mortgage loan financing
Liabilities:
Fair Value	693,055		595,778
Level 3 | CLO debt
Liabilities:
Fair Value	688,479
Level 3 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair Value	3,107
Level 3 | Borrowings from the FHLB
Liabilities:
Fair Value	1,369,544		1,662,178
Level 3 | Senior unsecured notes
Liabilities:
Fair Value	1,187,187		550,562
Level 3 | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair Value	3,292,035		2,018,973
Level 3 | Mortgage loan receivables held for sale
Assets:
Fair Value	236,428		359,897
Level 3 | FHLB stock
Assets:
Fair Value	77,915		77,915
Recurring
Assets:
Fair Value	1,107,405		2,105,965
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	54,160		100,400
Fair Value	2,606		3,446
Recurring | CMBS
Assets:
Outstanding Face Amount	945,167		1,676,680
Fair Value	953,499		1,701,395
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	3,140,297		8,160,458
Fair Value	113,071		342,171
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	172,916		478,577
Fair Value	4,477		16,821
Recurring | Agency securities
Assets:
Outstanding Face Amount	720		774
Fair Value	728		780
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	33,745		38,327
Fair Value	34,742		39,780
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	594,140		847,000
Fair Value	888		5,018
Recurring | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Provision for loan losses	(4,000)		(4,000)
Recurring | Level 1
Assets:
Fair Value	0		0
Recurring | Level 1 | Nonhedge derivatives
Liabilities:
Fair Value	0		0
Recurring | Level 1 | CMBS
Assets:
Fair Value	0		0
Recurring | Level 1 | CMBS interest-only
Assets:
Fair Value	0		0
Recurring | Level 1 | GNMA interest-only
Assets:
Fair Value	0		0
Recurring | Level 1 | Agency securities
Assets:
Fair Value	0		0
Recurring | Level 1 | GNMA permanent securities
Assets:
Fair Value	0		0
Recurring | Level 1 | Nonhedge derivatives
Assets:
Fair Value	0		0
Recurring | Level 2
Assets:
Fair Value	888		5,018
Recurring | Level 2 | Nonhedge derivatives
Liabilities:
Fair Value	2,606		3,446
Recurring | Level 2 | CMBS
Assets:
Fair Value	0		0
Recurring | Level 2 | CMBS interest-only
Assets:
Fair Value	0		0
Recurring | Level 2 | GNMA interest-only
Assets:
Fair Value	0		0
Recurring | Level 2 | Agency securities
Assets:
Fair Value	0		0
Recurring | Level 2 | GNMA permanent securities
Assets:
Fair Value	0		0
Recurring | Level 2 | Nonhedge derivatives
Assets:
Fair Value	888		5,018
Recurring | Level 3
Assets:
Fair Value	1,106,517		2,100,947
Recurring | Level 3 | Nonhedge derivatives
Liabilities:
Fair Value	0		0
Recurring | Level 3 | CMBS
Assets:
Fair Value	953,499		1,701,395
Recurring | Level 3 | CMBS interest-only
Assets:
Fair Value	113,071		342,171
Recurring | Level 3 | GNMA interest-only
Assets:
Fair Value	4,477		16,821
Recurring | Level 3 | Agency securities
Assets:
Fair Value	728		780
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair Value	34,742		39,780
Recurring | Level 3 | Nonhedge derivatives
Assets:
Fair Value	$ 0		$ 0

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