FAIR VALUE OF FINANCIAL INSTRUMENTS - Summary of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Fair Value	$ 2,984,372	$ 2,452,785
Liabilities:
Fair Value	4,399,236	3,940,704
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	549,892	629,430
Fair Value	549,892	629,430
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	489,464	477,756
Fair Value	489,464	477,756
Revolving credit facility
Liabilities:
Outstanding Face Amount	168,000	25,000
Fair Value	168,000	25,000
Mortgage loan financing
Liabilities:
Outstanding Face Amount	589,152	589,152
Fair Value	594,012	595,778
Mortgage Loan Receivable Financing
Liabilities:
Outstanding Face Amount	57,038	0
Fair Value	57,038	0
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,475,500	1,660,000
Fair Value	1,476,686	1,662,178
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,057,732	563,872
Fair Value	1,064,144	550,562
Mortgage loan receivables held for investment, at amortized cost
Assets:
Outstanding Face Amount	2,317,221	2,011,309
Fair Value	2,321,245	2,014,973
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	520,679	360,518
Fair Value	585,212	359,897
FHLB stock
Assets:
Outstanding Face Amount	77,915	77,915
Fair Value	77,915	77,915
Level 3
Assets:
Fair Value	2,984,372	2,452,785
Liabilities:
Fair Value	4,399,236	3,940,704
Level 3 | Repurchase agreements - short-term
Liabilities:
Fair Value	549,892	629,430
Level 3 | Repurchase agreements - long-term
Liabilities:
Fair Value	489,464	477,756
Level 3 | Revolving credit facility
Liabilities:
Fair Value	168,000	25,000
Level 3 | Mortgage loan financing
Liabilities:
Fair Value	594,012	595,778
Level 3 | Mortgage Loan Receivable Financing
Liabilities:
Fair Value	57,038	0
Level 3 | Borrowings from the FHLB
Liabilities:
Fair Value	1,476,686	1,662,178
Level 3 | Senior unsecured notes
Liabilities:
Fair Value	1,064,144	550,562
Level 3 | Mortgage loan receivables held for investment, at amortized cost
Assets:
Fair Value	2,321,245	2,014,973
Level 3 | Mortgage loan receivables held for sale
Assets:
Fair Value	585,212	359,897
Level 3 | FHLB stock
Assets:
Fair Value	77,915	77,915
Recurring
Assets:
Fair Value	1,702,088	2,105,965
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	1,001,200	100,400
Fair Value	4,207	3,446
Recurring | CMBS
Assets:
Outstanding Face Amount	1,306,653	1,676,680
Fair Value	1,325,250	1,701,395
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	7,801,370	8,160,458
Fair Value	320,865	342,171
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	457,597	478,577
Fair Value	15,625	16,821
Recurring | Agency securities
Assets:
Outstanding Face Amount	760	774
Fair Value	771	780
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	38,029	38,327
Fair Value	39,469	39,780
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	97,100	847,000
Fair Value	108	5,018
Recurring | Level 2
Assets:
Fair Value	108	5,018
Recurring | Level 2 | Nonhedge derivatives
Liabilities:
Fair Value	4,207	3,446
Recurring | Level 2 | Nonhedge derivatives
Assets:
Fair Value	108	5,018
Recurring | Level 3
Assets:
Fair Value	1,701,980	2,100,947
Recurring | Level 3 | CMBS
Assets:
Fair Value	1,325,250	1,701,395
Recurring | Level 3 | CMBS interest-only
Assets:
Fair Value	320,865	342,171
Recurring | Level 3 | GNMA interest-only
Assets:
Fair Value	15,625	16,821
Recurring | Level 3 | Agency securities
Assets:
Fair Value	771	780
Recurring | Level 3 | GNMA permanent securities
Assets:
Fair Value	$ 39,469	$ 39,780