FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Assets:
Fair Value	$ 3,619,312	$ 3,451,739
Current expected credit losses	(49,457)	(20,500)
Liabilities:
Fair Value	$ 4,842,833	$ 4,943,914
Total mortgage loan receivables held for investment, net, at amortized cost
Liabilities:
Period of short interest rate reset risk	30 days	30 days
CLO debt
Liabilities:
Period of short interest rate reset risk	30 days	30 days
Recurring
Assets:
Fair Value	$ 1,919,217	$ 1,709,586
Recurring \| CMBS \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	1,943,507	1,640,597
Amortized Cost Basis/Purchase Price	1,943,359	1,640,905
Fair Value	$ 1,868,149	$ 1,644,322
Liabilities:
Financial instruments, measurement input	0.0221	0.0308
Weighted average remaining maturity/duration	2 years 3 months 21 days	2 years 4 months 28 days
Recurring \| CMBS interest-only \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 1,550,358	$ 1,559,160
Amortized Cost Basis/Purchase Price	26,800	28,553
Fair Value	$ 27,835	$ 29,146
Liabilities:
Financial instruments, measurement input	0.0299	0.0304
Weighted average remaining maturity/duration	2 years 5 months 15 days	2 years 6 months 10 days
Recurring \| GNMA interest-only \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 105,009	$ 109,783
Amortized Cost Basis/Purchase Price	1,661	1,982
Fair Value	$ 1,607	$ 1,851
Liabilities:
Financial instruments, measurement input	0.0486	0.0459
Weighted average remaining maturity/duration	3 years 18 days	2 years 9 months 7 days
Recurring \| Agency securities \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 621	$ 629
Amortized Cost Basis/Purchase Price	631	640
Fair Value	$ 628	$ 637
Liabilities:
Financial instruments, measurement input	0.0170	0.0173
Weighted average remaining maturity/duration	1 year 8 months 12 days	1 year 9 months 29 days
Recurring \| GNMA permanent securities \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	$ 31,159	$ 31,461
Amortized Cost Basis/Purchase Price	31,345	31,681
Fair Value	$ 32,211	$ 32,369
Liabilities:
Financial instruments, measurement input	0.0350	0.0317
Weighted average remaining maturity/duration	2 years 7 months 9 days	1 year 11 months 4 days
Recurring \| Equity securities \| Internal Model Third Party Inputs Valuation Technique
Assets:
Amortized Cost Basis/Purchase Price	$ 598	$ 12,848
Fair Value	197	12,980
Recurring \| Total mortgage loan receivables held for investment, net, at amortized cost
Assets:
Current expected credit losses	(49,457)
Recurring \| Total mortgage loan receivables held for investment, net, at amortized cost \| Discounted Cash Flow
Assets:
Outstanding Face Amount	3,453,893	3,277,596
Amortized Cost Basis/Purchase Price	3,432,779	3,257,036
Fair Value	3,447,851	$ 3,273,219
Current expected credit losses	$ (49,457)
Liabilities:
Financial instruments, measurement input	0.0686	0.0694
Weighted average remaining maturity/duration	1 year 3 months 14 days	1 year 5 months 4 days
Recurring \| Provisions For Loan Losses
Assets:
Current expected credit losses	$ (22)	$ (20,500)
Recurring \| Provisions For Loan Losses \| Internal Model Third Party Inputs Valuation Technique
Assets:
Current expected credit losses	(22)
Recurring \| Mortgage loan receivables held for sale \| Internal Model Third Party Inputs Valuation Technique
Assets:
Outstanding Face Amount	154,833	122,748
Amortized Cost Basis/Purchase Price	146,713	122,325
Fair Value	$ 146,961	$ 124,989
Liabilities:
Financial instruments, measurement input	0.0394	0.0420
Weighted average remaining maturity/duration	9 years 11 months 15 days	9 years 11 months 26 days
Recurring \| FHLB stock \| FHLB stock
Assets:
Outstanding Face Amount	$ 61,619	$ 61,619
Amortized Cost Basis/Purchase Price	61,619	61,619
Fair Value	$ 61,619	$ 61,619
Liabilities:
Financial instruments, measurement input	0.0425	0.0475
Recurring \| Nonhedge derivatives \| Counterparty Quotations Valuation Technique
Assets:
Nonhedge derivative assets	$ 121,400	$ 340,200
Fair Value	$ 950	$ 693
Liabilities:
Weighted average remaining maturity/duration	3 months	3 months
Recurring \| Repurchase agreements - short-term \| Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 1,626,706	$ 1,781,253
Amortized Cost Basis/Purchase Price	1,626,706	1,781,253
Fair Value	$ 1,626,706	$ 1,781,253
Financial instruments, measurement input	0.0287	0.0250
Weighted average remaining maturity/duration	2 months 12 days	2 months 8 days
Recurring \| Repurchase agreements - long-term \| Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 100,094	$ 34,681
Amortized Cost Basis/Purchase Price	100,094	34,681
Fair Value	$ 100,094	$ 34,681
Financial instruments, measurement input	0.0248	0.0281
Weighted average remaining maturity/duration	1 year 4 months 9 days	1 year 4 months 28 days
Recurring \| Revolving Credit Facility \| Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 266,430
Amortized Cost Basis/Purchase Price	266,430
Fair Value	$ 266,430
Financial instruments, measurement input	0.0383
Weighted average remaining maturity/duration	10 months 13 days
Recurring \| Mortgage loan financing \| Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 801,515	$ 807,854
Amortized Cost Basis/Purchase Price	806,153	812,606
Fair Value	$ 830,648	$ 838,766
Financial instruments, measurement input	0.0490	0.0491
Weighted average remaining maturity/duration	1 year 6 months 10 days	1 year 6 months 3 days
Recurring \| Borrowings from the FHLB \| Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 1,007,581	$ 1,073,500
Amortized Cost Basis/Purchase Price	1,007,581	1,073,500
Fair Value	$ 1,012,997	$ 1,080,354
Financial instruments, measurement input	0.0208	0.0233
Weighted average remaining maturity/duration	1 year 11 months 1 day	2 years 29 days
Recurring \| Senior unsecured notes \| Broker Quotations Pricing Services Valuation Technique
Liabilities:
Outstanding Face Amount	$ 1,891,897	$ 1,166,201
Amortized Cost Basis/Purchase Price	1,874,056	1,157,833
Fair Value	$ 1,005,958	$ 1,208,860
Financial instruments, measurement input	0.0495	0.0539
Weighted average remaining maturity/duration	4 years 6 months	3 years 3 months 10 days
Recurring \| Nonhedge derivatives \| Counterparty Quotations Valuation Technique
Liabilities:
Nonhedge derivative liabilities	$ 69,571	$ 69,571
Fair Value	$ 0	$ 0
Weighted average remaining maturity/duration	1 month 9 days	4 months 9 days

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