FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Fair value of assets	$ 3,947,492	$ 3,602,378
Provision for loan losses	(7,300)	(4,000)
Liabilities:
Fair value of liabilities	4,657,530	4,414,782
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	563,869	371,427
Fair value of liabilities	563,870	371,427
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	256,093	101,983
Fair value of liabilities	256,093	101,983
Revolving credit facility
Liabilities:
Outstanding Face Amount		0
Fair value of liabilities		0
Mortgage loan financing
Liabilities:
Outstanding Face Amount	784,366	692,394
Fair value of liabilities	752,120	693,055
CLO debt
Liabilities:
Outstanding Face Amount	685,416
Fair value of liabilities	685,416
Participation Financing - Mortgage Loan Receivable
Liabilities:
Outstanding Face Amount	2,647	688,479
Fair value of liabilities	2,647	688,479
Liability for transfers not considered sales
Liabilities:
Outstanding Face Amount		3,107
Fair value of liabilities		3,107
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,270,000	1,370,000
Fair value of liabilities	1,253,440	1,369,544
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,166,201	1,166,201
Fair value of liabilities	1,143,944	1,187,187
Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Outstanding Face Amount	3,788,546	3,300,709
Fair value of assets	3,766,295	3,292,035
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	108,340	232,527
Fair value of assets	110,582	236,428
FHLB stock
Assets:
Outstanding Face Amount	77,915	77,915
Fair value of assets	77,915	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 | Revolving credit facility
Liabilities:
Fair value of liabilities		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 | Liability for transfers not considered sales
Liabilities:
Fair value of liabilities		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 | 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 | Revolving credit facility
Liabilities:
Fair value of liabilities		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 | Liability for transfers not considered sales
Liabilities:
Fair value of liabilities		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 | 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	3,947,492	3,602,378
Provision for loan losses	(7,300)	(4,000)
Liabilities:
Fair value of liabilities	4,657,530	4,414,782
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair value of liabilities	563,870	371,427
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair value of liabilities	256,093	101,983
Level 3 | Revolving credit facility
Liabilities:
Fair value of liabilities		0
Level 3 | Mortgage loan financing
Liabilities:
Fair value of liabilities	752,120	693,055
Level 3 | CLO debt
Liabilities:
Fair value of liabilities	685,416
Level 3 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair value of liabilities	2,647	688,479
Level 3 | Liability for transfers not considered sales
Liabilities:
Fair value of liabilities		3,107
Level 3 | Borrowings from the FHLB
Liabilities:
Fair value of liabilities	1,253,440	1,369,544
Level 3 | Senior unsecured notes
Liabilities:
Fair value of liabilities	1,143,944	1,187,187
Level 3 | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Fair value of assets	3,766,295	3,292,035
Level 3 | Mortgage loan receivables held for sale
Assets:
Fair value of assets	110,582	236,428
Level 3 | FHLB stock
Assets:
Fair value of assets	77,915	77,915
Recurring
Assets:
Fair value of assets	1,107,018	1,107,405
Recurring | Nonhedge derivatives
Liabilities:
Nonhedge derivative liabilities	96,471	54,160
Fair value of liabilities	0	2,606
Recurring | CMBS
Assets:
Outstanding Face Amount	999,461	945,167
Fair value of assets	994,297	953,499
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	2,466,340	3,140,297
Fair value of assets	74,546	113,071
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	143,738	172,916
Fair value of assets	3,196	4,477
Recurring | Agency securities
Assets:
Outstanding Face Amount	696	720
Fair value of assets	686	728
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	33,196	33,745
Fair value of assets	33,633	34,742
Recurring | Nonhedge derivatives
Assets:
Nonhedge derivative assets	579,800	594,140
Fair value of assets	660	888
Recurring | Level 1
Assets:
Fair value of assets	0	0
Recurring | Level 1 | Nonhedge derivatives
Liabilities:
Fair value of liabilities	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	0
Recurring | Level 1 | Nonhedge derivatives
Assets:
Fair value of assets	0	0
Recurring | Level 2
Assets:
Fair value of assets	660	888
Recurring | Level 2 | Nonhedge derivatives
Liabilities:
Fair value of liabilities	0	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 | Nonhedge derivatives
Assets:
Fair value of assets	660	888
Recurring | Level 3
Assets:
Fair value of assets	1,106,358	1,106,517
Recurring | Level 3 | Nonhedge derivatives
Liabilities:
Fair value of liabilities	0	0
Recurring | Level 3 | CMBS
Assets:
Fair value of assets	994,297	953,499
Recurring | Level 3 | CMBS interest-only
Assets:
Fair value of assets	74,546	113,071
Recurring | Level 3 | GNMA interest-only
Assets:
Fair value of assets	3,196	4,477
Recurring | Level 3 | Agency securities
Assets:
Fair value of assets	686	728
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair value of assets	33,633	34,742
Recurring | Level 3 | Nonhedge derivatives
Assets:
Fair value of assets	$ 0	$ 0