FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	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
Fair Value Assumptions:
Period of short interest rate reset risk	30 days	30 days
Revolving credit facility | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 76,000
Amortized Cost Basis	76,000
Fair Value	$ 76,000
Fair Value Assumptions:
Weighted average yield %	5.68%
Weighted Average Remaining Maturity/Duration	22 days
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, net, at amortized cost
Assets:
Outstanding Face Amount	2,862,739	2,011,309
Fair Value	$ 2,853,389	$ 2,018,973
Fair Value Assumptions:
Period of short interest rate reset risk	30 days	30 days
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
Recurring
Assets:
Fair Value	1,099,039	2,105,965
Recurring | Repurchase agreements - short-term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	549,899	629,430
Amortized Cost Basis	549,899	629,430
Fair Value	$ 549,899	$ 629,430
Fair Value Assumptions:
Weighted average yield %	2.67%	2.10%
Weighted Average Remaining Maturity/Duration	16 days	5 days
Recurring | Repurchase agreements - long-term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 363,238	$ 477,756
Amortized Cost Basis	363,238	477,756
Fair Value	$ 363,238	$ 477,756
Fair Value Assumptions:
Weighted average yield %	2.98%	2.00%
Weighted Average Remaining Maturity/Duration	2 years 7 months 20 days	1 year 8 months 12 days
Recurring | Revolving credit facility | Discounted Cash Flow
Liabilities:
Outstanding Face Amount		$ 25,000
Amortized Cost Basis		25,000
Fair Value		$ 25,000
Fair Value Assumptions:
Weighted average yield %		3.16%
Weighted Average Remaining Maturity/Duration		3 days
Recurring | Mortgage loan financing | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 589,152	$ 589,152
Amortized Cost Basis	587,490	590,106
Fair Value	$ 595,106	$ 595,778
Fair Value Assumptions:
Weighted average yield %	4.85%	4.85%
Weighted Average Remaining Maturity/Duration	6 years 4 months 24 days	7 years 1 month 24 days
Recurring | Participation Financing - Mortgage Loan Receivable | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 3,368
Amortized Cost Basis	3,368
Fair Value	$ 3,368
Fair Value Assumptions:
Weighted average yield %	17.00%
Weighted Average Remaining Maturity/Duration	5 days
Recurring | Borrowings from the FHLB | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 1,464,000	$ 1,660,000
Amortized Cost Basis	1,464,000	1,660,000
Fair Value	$ 1,465,922	$ 1,662,178
Fair Value Assumptions:
Weighted average yield %	1.50%	1.12%
Weighted Average Remaining Maturity/Duration	2 years 6 months 7 days	2 years 5 months 1 day
Recurring | Senior unsecured notes | Broker quotations, pricing services
Liabilities:
Outstanding Face Amount	$ 1,166,201	$ 563,872
Amortized Cost Basis	1,152,552	559,847
Fair Value	$ 787,191	$ 550,562
Fair Value Assumptions:
Weighted average yield %	5.39%	6.67%
Weighted Average Remaining Maturity/Duration	5 years 6 months 10 days	2 years 9 months 21 days
Recurring | Liability for transfers not considered sales | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 636,256
Amortized Cost Basis	631,480
Fair Value	$ 631,480
Fair Value Assumptions:
Weighted average yield %	4.37%
Weighted Average Remaining Maturity/Duration	8 years 4 months 17 days
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	$ 88,700	$ 100,400
Fair Value	2,711	3,446
Recurring | Nonhedge derivatives | Counterparty quotations
Liabilities:
Derivative liability face amount	88,700	100,400
Fair Value	$ 2,711	$ 3,446
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	2 years 9 months 3 days	3 years 2 months 15 days
Recurring | CMBS
Assets:
Outstanding Face Amount	$ 934,559	$ 1,676,680
Fair Value	950,029	1,701,395
Recurring | CMBS | Internal model, third-party inputs
Assets:
Outstanding Face Amount	934,559	1,676,680
Amortized Cost Basis	946,231	1,698,276
Fair Value	$ 950,029	$ 1,701,395
Fair Value Assumptions:
Weighted average yield %	2.73%	2.81%
Weighted Average Remaining Maturity/Duration	2 years 10 months 6 days	3 years 6 months 18 days
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	$ 3,308,063	$ 8,160,458
Fair Value	104,483	342,171
Recurring | CMBS interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	3,308,063	8,160,458
Amortized Cost Basis	102,986	343,534
Fair Value	$ 104,483	$ 342,171
Fair Value Assumptions:
Weighted average yield %	3.14%	3.45%
Weighted Average Remaining Maturity/Duration	2 years 10 months 17 days	2 years 11 months 26 days
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	$ 279,567	$ 478,577
Fair Value	7,999	16,821
Recurring | GNMA interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	279,567	478,577
Amortized Cost Basis	9,138	18,994
Fair Value	$ 7,999	$ 16,821
Fair Value Assumptions:
Weighted average yield %	5.99%	4.19%
Weighted Average Remaining Maturity/Duration	4 years 4 months 9 days	4 years 5 months 8 days
Recurring | Agency securities
Assets:
Outstanding Face Amount	$ 731	$ 774
Fair Value	742	780
Recurring | Agency securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	731	774
Amortized Cost Basis	754	802
Fair Value	$ 742	$ 780
Fair Value Assumptions:
Weighted average yield %	1.82%	1.29%
Weighted Average Remaining Maturity/Duration	3 years 29 days	3 years 3 months 7 days
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	$ 34,014	$ 38,327
Fair Value	35,218	39,780
Recurring | GNMA permanent securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	34,014	38,327
Amortized Cost Basis	34,675	39,145
Fair Value	$ 35,218	$ 39,780
Fair Value Assumptions:
Weighted average yield %	3.63%	3.80%
Weighted Average Remaining Maturity/Duration	5 years 9 months 25 days	10 years 3 months 18 days
Recurring | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Provision for loan losses	$ (4,000)	$ (4,000)
Recurring | Mortgage loan receivables held for investment, net, at amortized cost | Discounted Cash Flow
Assets:
Outstanding Face Amount	2,862,739	2,011,309
Amortized Cost Basis	2,846,940	2,000,095
Fair Value	$ 2,853,389	$ 2,018,973
Fair Value Assumptions:
Weighted average yield %	7.06%	7.17%
Weighted Average Remaining Maturity/Duration	1 year 8 months 15 days	1 year 7 months 28 days
Recurring | Mortgage loans transferred but not considered sold | Discounted Cash Flow
Assets:
Outstanding Face Amount	$ 600,222
Amortized Cost Basis	598,525
Fair Value	$ 610,829
Fair Value Assumptions:
Weighted average yield %	4.92%
Weighted Average Remaining Maturity/Duration	8 years 4 months 17 days
Recurring | Mortgage loan receivables held for sale | Internal model, third-party inputs
Assets:
Outstanding Face Amount	$ 526,085	$ 360,518
Amortized Cost Basis	522,961	357,882
Fair Value	$ 540,510	$ 359,897
Fair Value Assumptions:
Weighted average yield %	4.79%	4.20%
Weighted Average Remaining Maturity/Duration	8 years 7 months 17 days	4 years 6 months 18 days
Recurring | FHLB stock | Put Value
Assets:
Outstanding Face Amount	$ 77,915	$ 77,915
Amortized Cost Basis	77,915	77,915
Fair Value	$ 77,915	$ 77,915
Fair Value Assumptions:
Weighted average yield %	4.25%	4.25%
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	$ 616,200	$ 847,000
Fair Value	568	5,018
Recurring | Nonhedge derivatives | Counterparty quotations
Assets:
Derivative asset face amount	616,200	847,000
Fair Value	$ 568	$ 5,018
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	8 days	7 days