FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Assets:
Fair Value		$ 3,564,681	$ 3,615,097
Provision for loan losses	[1]	(17,900)	(4,000)
Carrying Value		1,398,576	1,106,517
Liabilities:
Fair Value		4,401,295	4,414,782
Recurring
Assets:
Fair Value		1,397,918	1,094,687
CMBS
Assets:
Carrying Value		1,252,640	953,499
CMBS | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		1,258,819	945,167
Amortized Cost Basis/Purchase Price		1,257,801	$ 954,397
Fair Value		$ 1,252,640
Debt securities, available-for-sale, measurement input		0.0314	0.0279
Liabilities:
Weighted average remaining maturity/duration		2 years 3 months 29 days	2 years 10 months 20 days
CMBS interest-only
Assets:
Carrying Value		$ 55,691	$ 113,071
CMBS interest-only | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		2,373,936	3,140,297
Amortized Cost Basis/Purchase Price		55,534	$ 112,609
Fair Value		$ 55,691
Debt securities, available-for-sale, measurement input		0.0280	0.0316
Liabilities:
Weighted average remaining maturity/duration		2 years 8 months 8 days	3 years 29 days
GNMA interest-only
Assets:
Carrying Value		$ 2,648	$ 4,477
GNMA interest-only | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		135,932	172,916
Amortized Cost Basis/Purchase Price		2,862	$ 5,245
Fair Value		$ 2,648
Debt securities, available-for-sale, measurement input		0.0630	0.0670
Liabilities:
Weighted average remaining maturity/duration		4 years 1 month 9 days	4 years 2 months 4 days
Agency securities
Assets:
Carrying Value		$ 662	$ 728
Agency securities | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		668	720
Amortized Cost Basis/Purchase Price		682	$ 743
Fair Value		$ 662
Debt securities, available-for-sale, measurement input		0.0183	0.0180
Liabilities:
Weighted average remaining maturity/duration		2 years 4 months 9 days	2 years 11 months 8 days
GNMA permanent securities
Assets:
Carrying Value		$ 33,064	$ 34,742
GNMA permanent securities | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		32,633	33,745
Amortized Cost Basis/Purchase Price		32,889	$ 34,386
Fair Value		$ 33,064
Debt securities, available-for-sale, measurement input		0.0376	0.0362
Liabilities:
Weighted average remaining maturity/duration		5 years 10 days	5 years 7 months 27 days
Corporate bonds
Assets:
Carrying Value		$ 53,871
Corporate bonds | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		55,305
Amortized Cost Basis/Purchase Price		54,257
Fair Value		$ 53,871
Debt securities, available-for-sale, measurement input		0.0504
Liabilities:
Weighted average remaining maturity/duration		2 years 6 months 3 days
Common Stock | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Amortized Cost Basis/Purchase Price		$ 13,154
Fair Value		$ 11,550
Mortgage loan receivables held for investment, net, at amortized cost
Liabilities:
Period of short interest rate reset risk		30 days	30 days
Mortgage loan receivables held for investment, net, at amortized cost | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		$ 3,340,381
Mortgage loan receivables held for investment, net, at amortized cost | Recurring | Discounted Cash Flow
Assets:
Outstanding Face Amount			$ 3,300,709
Amortized Cost Basis/Purchase Price		3,318,390	3,282,462
Fair Value		$ 3,324,588	$ 3,292,035
Debt securities, available-for-sale, measurement input		0.0784	0.0718
Liabilities:
Weighted average remaining maturity/duration		1 year 3 months 25 days	1 year 7 months 9 days
Provisions For Loan Losses | Recurring
Assets:
Provision for loan losses		$ (17,900)	$ (4,000)
Mortgage loan receivables held for sale | Recurring | Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount		181,905	232,527
Amortized Cost Basis/Purchase Price		182,439	230,180
Fair Value		$ 187,870	$ 236,428
Debt securities, available-for-sale, measurement input		0.0546	0.0488
Liabilities:
Weighted average remaining maturity/duration		9 years 9 months	8 years 2 months 1 day
FHLB stock | Recurring | FHLB stock
Assets:
Outstanding Face Amount		$ 57,915	$ 77,915
Amortized Cost Basis/Purchase Price		57,915	77,915
Fair Value		$ 57,915	$ 77,915
Debt securities, available-for-sale, measurement input		0.0450	0.0425
Nonhedge derivatives | Recurring | Counterparty Quotations Valuation Technique
Assets:
Nonhedge derivative assets			$ 594,140
Fair Value			$ 888
Liabilities:
Weighted average remaining maturity/duration			2 months 26 days
Repurchase agreements - short-term | Recurring | Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input		0.0342	0.0319
Liabilities:
Outstanding Face Amount		$ 436,957	$ 371,427
Amortized Cost Basis/Purchase Price		436,957	371,427
Fair Value		$ 436,957	$ 371,427
Weighted average remaining maturity/duration		2 months 23 days	4 months 6 days
Repurchase agreements - long-term | Recurring | Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input		0.0347	0.0262
Liabilities:
Outstanding Face Amount		$ 226,728	$ 101,983
Amortized Cost Basis/Purchase Price		226,728	101,983
Fair Value		$ 226,728	$ 101,983
Weighted average remaining maturity/duration		1 year 8 months 23 days	2 years 7 months 20 days
Mortgage loan financing | Recurring | Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input		0.0509	0.0491
Liabilities:
Outstanding Face Amount		$ 738,825	$ 692,394
Amortized Cost Basis/Purchase Price		743,902	692,696
Fair Value		$ 735,662	$ 693,055
Weighted average remaining maturity/duration		2 years 7 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		0.0441	0.0340
Liabilities:
Outstanding Face Amount		$ 601,543	$ 688,479
Amortized Cost Basis/Purchase Price		601,543	688,479
Fair Value		$ 601,543	$ 688,479
Weighted average remaining maturity/duration		9 years 4 months 24 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		$ 2,453	$ 3,107
Amortized Cost Basis/Purchase Price		2,453	3,107
Fair Value		$ 2,453	$ 3,107
Weighted average remaining maturity/duration		5 months 4 days	5 months 4 days
Borrowings from the FHLB | Recurring | Discounted Cash Flow
Assets:
Debt securities, available-for-sale, measurement input		0.0255	0.0161
Liabilities:
Outstanding Face Amount		$ 1,286,000	$ 1,370,000
Amortized Cost Basis/Purchase Price		1,286,000	1,370,000
Fair Value		$ 1,286,664	$ 1,369,544
Weighted average remaining maturity/duration		2 years 5 months 15 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/Purchase Price		1,154,991	1,152,134
Fair Value		$ 1,111,288	$ 1,187,187
Weighted average remaining maturity/duration		4 years 3 months 10 days	5 years 3 months 10 days
Nonhedge derivatives | Recurring | Counterparty Quotations Valuation Technique
Liabilities:
Nonhedge derivative liabilities		$ 578,971	$ 54,160
Fair Value		$ 975	$ 2,606
Weighted average remaining maturity/duration		3 months	2 years 5 months 8 days

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