FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Fair Value	$ 4,078,643	$ 2,452,785
Provision for loan losses	(4,000)	(4,000)
Liabilities:
Fair Value	4,472,204	3,940,704
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	549,899	629,430
Fair Value	549,899	629,430
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	363,238	477,756
Fair Value	363,238	477,756
Revolving credit facility
Liabilities:
Outstanding Face Amount	76,000	25,000
Fair Value	76,000	25,000
Mortgage loan financing
Liabilities:
Outstanding Face Amount	589,152	589,152
Fair Value	595,106	595,778
Participation Financing - Mortgage Loan Receivable
Liabilities:
Outstanding Face Amount	3,368
Fair Value	3,368
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,464,000	1,660,000
Fair Value	1,465,922	1,662,178
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,166,201	563,872
Fair Value	787,191	550,562
Liability for transfers not considered sales
Liabilities:
Outstanding Face Amount	636,256
Fair Value	631,480
Mortgage loan receivables held for investment, at amortized cost
Assets:
Outstanding Face Amount	2,862,739	2,011,309
Fair Value	2,853,389	2,018,973
Mortgage loans transferred but not considered sold
Assets:
Outstanding Face Amount	600,222
Fair Value	610,829
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	526,085	360,518
Fair Value	540,510	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	0
Level 1 | Mortgage loan financing
Liabilities:
Fair Value	0	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 | Liability for transfers not considered sales
Liabilities:
Fair Value	0
Level 1 | Mortgage loan receivables held for investment, at amortized cost
Assets:
Fair Value	0	0
Level 1 | Mortgage loans transferred but not considered sold
Assets:
Fair Value	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	0
Level 2 | Mortgage loan financing
Liabilities:
Fair Value	0	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 | Liability for transfers not considered sales
Liabilities:
Fair Value	0
Level 2 | Mortgage loan receivables held for investment, at amortized cost
Assets:
Fair Value	0	0
Level 2 | Mortgage loans transferred but not considered sold
Assets:
Fair Value	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	4,078,643	2,452,785
Provision for loan losses	(4,000)	(4,000)
Liabilities:
Fair Value	4,472,204	3,940,704
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair Value	549,899	629,430
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair Value	363,238	477,756
Level 3 | Revolving credit facility
Liabilities:
Fair Value	76,000	25,000
Level 3 | Mortgage loan financing
Liabilities:
Fair Value	595,106	595,778
Level 3 | Participation Financing - Mortgage Loan Receivable
Liabilities:
Fair Value	3,368
Level 3 | Borrowings from the FHLB
Liabilities:
Fair Value	1,465,922	1,662,178
Level 3 | Senior unsecured notes
Liabilities:
Fair Value	787,191	550,562
Level 3 | Liability for transfers not considered sales
Liabilities:
Fair Value	631,480
Level 3 | Mortgage loan receivables held for investment, at amortized cost
Assets:
Fair Value	2,853,389	2,018,973
Level 3 | Mortgage loans transferred but not considered sold
Assets:
Fair Value	610,829
Level 3 | Mortgage loan receivables held for sale
Assets:
Fair Value	540,510	359,897
Level 3 | FHLB stock
Assets:
Fair Value	77,915	77,915
Recurring
Assets:
Fair Value	1,099,039	2,105,965
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	88,700	100,400
Fair Value	2,711	3,446
Recurring | CMBS
Assets:
Outstanding Face Amount	934,559	1,676,680
Fair Value	950,029	1,701,395
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	3,308,063	8,160,458
Fair Value	104,483	342,171
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	279,567	478,577
Fair Value	7,999	16,821
Recurring | Agency securities
Assets:
Outstanding Face Amount	731	774
Fair Value	742	780
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	34,014	38,327
Fair Value	35,218	39,780
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	616,200	847,000
Fair Value	568	5,018
Recurring | Mortgage loan receivables held for investment, 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	568	5,018
Recurring | Level 2 | Nonhedge derivatives
Liabilities:
Fair Value	2,711	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	568	5,018
Recurring | Level 3
Assets:
Fair Value	1,098,471	2,100,947
Recurring | Level 3 | Nonhedge derivatives
Liabilities:
Fair Value	0	0
Recurring | Level 3 | CMBS
Assets:
Fair Value	950,029	1,701,395
Recurring | Level 3 | CMBS interest-only
Assets:
Fair Value	104,483	342,171
Recurring | Level 3 | GNMA interest-only
Assets:
Fair Value	7,999	16,821
Recurring | Level 3 | Agency securities
Assets:
Fair Value	742	780
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair Value	35,218	39,780
Recurring | Level 3 | Nonhedge derivatives
Assets:
Fair Value	$ 0	$ 0