FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Assets:
Fair Value	$ 3,947,492	$ 3,602,378
Provision for loan losses	(7,300)	(4,000)
Liabilities:
Fair Value	4,657,530	4,414,782
Recurring
Assets:
Fair Value	1,107,018	1,107,405
CMBS \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	999,461	945,167
Amortized Cost Basis	1,005,795	954,397
Fair Value	$ 994,297	$ 953,499
Debt securities, available-for-sale, measurement input	0.0292	0.0279
Liabilities:
Weighted average remaining maturity/duration	2 years 10 months 6 days	2 years 10 months 20 days
CMBS interest-only \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 2,466,340	$ 3,140,297
Amortized Cost Basis	74,383	112,609
Fair Value	$ 74,546	$ 113,071
Debt securities, available-for-sale, measurement input	0.0325	0.0316
Liabilities:
Weighted average remaining maturity/duration	2 years 10 months 13 days	3 years 29 days
GNMA interest-only \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 143,738	$ 172,916
Amortized Cost Basis	3,651	5,245
Fair Value	$ 3,196	$ 4,477
Debt securities, available-for-sale, measurement input	0.0719	0.0670
Liabilities:
Weighted average remaining maturity/duration	4 years 1 month 6 days	4 years 2 months 4 days
Agency securities \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 696	$ 720
Amortized Cost Basis	712	743
Fair Value	$ 686	$ 728
Debt securities, available-for-sale, measurement input	0.0193	0.0180
Liabilities:
Weighted average remaining maturity/duration	2 years 7 months 20 days	2 years 11 months 8 days
GNMA permanent securities \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 33,196	$ 33,745
Amortized Cost Basis	33,471	34,386
Fair Value	$ 33,633	$ 34,742
Debt securities, available-for-sale, measurement input	0.0377	0.0362
Liabilities:
Weighted average remaining maturity/duration	5 years 4 months 6 days	5 years 7 months 27 days
Mortgage loan receivables held for investment, net, at amortized cost
Liabilities:
Period of short interest rate reset risk	30 days
Mortgage loan receivables held for investment, net, at amortized cost \| Recurring
Assets:
Provision for loan losses	$ (7,300)	$ (4,000)
Mortgage loan receivables held for investment, net, at amortized cost \| Recurring \| Discounted Cash Flow
Assets:
Outstanding Face Amount	3,788,546	3,300,709
Amortized Cost Basis	3,764,172	3,282,462
Fair Value	$ 3,766,295	$ 3,292,035
Debt securities, available-for-sale, measurement input	0.0770	0.0718
Liabilities:
Weighted average remaining maturity/duration	1 year 5 months 23 days	1 year 7 months 9 days
Mortgage loan receivables held for sale \| Recurring \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 108,340	$ 232,527
Amortized Cost Basis	107,744	230,180
Fair Value	$ 110,582	$ 236,428
Debt securities, available-for-sale, measurement input	0.0524	0.0488
Liabilities:
Weighted average remaining maturity/duration	9 years 9 months 14 days	8 years 2 months 1 day
FHLB stock \| Recurring \| FHLB stock
Assets:
Outstanding Face Amount	$ 77,915	$ 77,915
Amortized Cost Basis	77,915	77,915
Fair Value	$ 77,915	$ 77,915
Debt securities, available-for-sale, measurement input	0.0425	0.0425
Nonhedge derivatives \| Recurring \| Counterparty Quotations Valuation Technique
Assets:
Nonhedge derivative assets	$ 579,800	$ 594,140
Fair Value	$ 660	$ 888
Liabilities:
Weighted average remaining maturity/duration	3 months	2 months 26 days
Repurchase agreements - short-term \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.0351	0.0319
Liabilities:
Outstanding Face Amount	$ 563,870	$ 371,427
Amortized Cost Basis	563,870	371,427
Fair Value	$ 563,870	$ 371,427
Weighted average remaining maturity/duration	7 months 6 days	4 months 6 days
Repurchase agreements - long-term \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.0272	0.0262
Liabilities:
Outstanding Face Amount	$ 256,093	$ 101,983
Amortized Cost Basis	256,093	101,983
Fair Value	$ 256,093	$ 101,983
Weighted average remaining maturity/duration	1 year 8 months 8 days	2 years 7 months 20 days
Mortgage loan financing \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.0501	0.0491
Liabilities:
Outstanding Face Amount	$ 784,366	$ 692,394
Amortized Cost Basis	770,880	692,696
Fair Value	$ 752,120	$ 693,055
Weighted average remaining maturity/duration	2 years 10 months 9 days	6 years 9 months 21 days
CLO debt
Liabilities:
Period of short interest rate reset risk	30 days	30 days
CLO debt \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.0395	0.0340
Liabilities:
Outstanding Face Amount	$ 685,416	$ 688,479
Amortized Cost Basis	685,416	688,479
Fair Value	$ 685,416	$ 688,479
Weighted average remaining maturity/duration	10 years 2 months 26 days	10 years 9 months 7 days
Participation Financing - Mortgage Loan Receivable \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.1700	0.1700
Liabilities:
Outstanding Face Amount	$ 2,647	$ 3,107
Amortized Cost Basis	2,647	3,107
Fair Value	$ 2,647	$ 3,107
Weighted average remaining maturity/duration	5 months 8 days	5 months 4 days
Borrowings from the FHLB \| Recurring \| Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input	0.0207	0.0161
Liabilities:
Outstanding Face Amount	$ 1,270,000	$ 1,370,000
Amortized Cost Basis	1,270,000	1,370,000
Fair Value	$ 1,253,440	$ 1,369,544
Weighted average remaining maturity/duration	2 years 5 months 4 days	2 years 5 months 26 days
Senior unsecured notes \| Recurring \| Broker Quotations Pricing Services Valuation Technique
Assets:
Debt securities, available-for-sale, measurement input	0.0539	0.0539
Liabilities:
Outstanding Face Amount	$ 1,166,201	$ 1,166,201
Amortized Cost Basis	1,153,543	1,152,134
Fair Value	$ 1,143,944	$ 1,187,187
Weighted average remaining maturity/duration	4 years 9 months 10 days	5 years 3 months 10 days
Nonhedge derivatives \| Recurring \| Counterparty Quotations Valuation Technique
Liabilities:
Nonhedge derivative liabilities	$ 96,471	$ 54,160
Fair Value	$ 0	$ 2,606
Weighted average remaining maturity/duration	1 year 10 months 6 days	2 years 5 months 8 days

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