FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018		Dec. 31, 2017
Assets:
Fair Value	$ 3,882,656		$ 3,602,378
Provision for loan losses	(7,000)	[1]	(4,000)
Liabilities:
Fair Value	4,586,037		4,414,782
Repurchase agreements - short-term
Liabilities:
Outstanding Face Amount	491,939		371,427
Fair Value	491,939		371,427
Repurchase agreements - long-term
Liabilities:
Outstanding Face Amount	262,438		101,983
Fair Value	262,438		101,983
Mortgage loan financing
Liabilities:
Outstanding Face Amount	695,163		692,394
Fair Value	668,790		693,055
CLO debt
Liabilities:
Outstanding Face Amount	683,095
Fair Value	683,095
Participation Financing - Mortgage Loan Receivable
Liabilities:
Outstanding Face Amount	2,815		688,479
Fair Value	2,815		688,479
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,348,000		1,370,000
Fair Value	1,326,100		1,369,544
Senior unsecured notes
Liabilities:
Outstanding Face Amount	1,166,201		1,166,201
Fair Value	1,150,860		1,187,187
Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Outstanding Face Amount	3,550,544		3,300,709
Fair Value	$ 3,532,369		3,292,035
Fair Value Assumptions:
Period of short interest rate reset risk	30 days
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	$ 274,090		232,527
Fair Value	279,372		236,428
FHLB stock
Assets:
Outstanding Face Amount	77,915		77,915
Fair Value	77,915		77,915
Recurring
Assets:
Fair Value	1,100,197		1,107,405
Recurring | Repurchase agreements - short-term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	491,939		371,427
Amortized Cost Basis	491,939		371,427
Fair Value	$ 491,939		$ 371,427
Fair Value Assumptions:
Weighted average yield %	3.25%		3.19%
Weighted Average Remaining Maturity/Duration	2 months 23 days		4 months 6 days
Recurring | Repurchase agreements - long-term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 262,438		$ 101,983
Amortized Cost Basis	262,438		101,983
Fair Value	$ 262,438		$ 101,983
Fair Value Assumptions:
Weighted average yield %	2.60%		2.62%
Weighted Average Remaining Maturity/Duration	1 year 10 months 20 days		2 years 7 months 20 days
Recurring | Mortgage loan financing | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 695,163		$ 692,394
Amortized Cost Basis	683,487		692,696
Fair Value	$ 668,790		$ 693,055
Fair Value Assumptions:
Weighted average yield %	4.91%		4.91%
Weighted Average Remaining Maturity/Duration	2 years 8 months 15 days		6 years 9 months 21 days
Recurring | CLO debt | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 683,095		$ 688,479
Amortized Cost Basis	683,095		688,479
Fair Value	$ 683,095		$ 688,479
Fair Value Assumptions:
Weighted average yield %	3.64%		3.40%
Weighted Average Remaining Maturity/Duration	10 years 5 months 19 days		10 years 9 months 7 days
Recurring | Participation Financing - Mortgage Loan Receivable | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 2,815		$ 3,107
Amortized Cost Basis	2,815		3,107
Fair Value	$ 2,815		$ 3,107
Fair Value Assumptions:
Weighted average yield %	17.00%		17.00%
Weighted Average Remaining Maturity/Duration	2 months 4 days		5 months 4 days
Recurring | Borrowings from the FHLB | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	$ 1,348,000		$ 1,370,000
Amortized Cost Basis	1,348,000		1,370,000
Fair Value	$ 1,326,100		$ 1,369,544
Fair Value Assumptions:
Weighted average yield %	1.82%		1.61%
Weighted Average Remaining Maturity/Duration	2 years 5 months 8 days		2 years 5 months 26 days
Recurring | Senior unsecured notes | Broker quotations, pricing services
Liabilities:
Outstanding Face Amount	$ 1,166,201		$ 1,166,201
Amortized Cost Basis	1,152,834		1,152,134
Fair Value	$ 1,150,860		$ 1,187,187
Fair Value Assumptions:
Weighted average yield %	5.39%		5.39%
Weighted Average Remaining Maturity/Duration	5 years 10 days		5 years 3 months 10 days
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount			$ 54,160
Fair Value			2,606
Recurring | Nonhedge derivatives | Counterparty quotations
Liabilities:
Derivative liability face amount			54,160
Fair Value			$ 2,606
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration			2 years 5 months 8 days
Recurring | CMBS
Assets:
Outstanding Face Amount	$ 983,104		$ 945,167
Fair Value	982,284		953,499
Recurring | CMBS | Internal model, third-party inputs
Assets:
Outstanding Face Amount	983,104		945,167
Amortized Cost Basis	991,436		954,397
Fair Value	$ 982,284		$ 953,499
Fair Value Assumptions:
Weighted average yield %	2.89%		2.79%
Weighted Average Remaining Maturity/Duration	2 years 11 months 4 days		2 years 10 months 20 days
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	$ 2,492,711		$ 3,140,297
Fair Value	79,035		113,071
Recurring | CMBS interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	2,492,711		3,140,297
Amortized Cost Basis	79,162		112,609
Fair Value	$ 79,035		$ 113,071
Fair Value Assumptions:
Weighted average yield %	3.46%		3.16%
Weighted Average Remaining Maturity/Duration	2 years 11 months 26 days		3 years 29 days
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	$ 165,307		$ 172,916
Fair Value	3,975		4,477
Recurring | GNMA interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	165,307		172,916
Amortized Cost Basis	4,540		5,245
Fair Value	$ 3,975		$ 4,477
Fair Value Assumptions:
Weighted average yield %	6.60%		6.70%
Weighted Average Remaining Maturity/Duration	4 years 14 days		4 years 2 months 4 days
Recurring | Agency securities
Assets:
Outstanding Face Amount	$ 706		$ 720
Fair Value	698		728
Recurring | Agency securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	706		720
Amortized Cost Basis	727		743
Fair Value	$ 698		$ 728
Fair Value Assumptions:
Weighted average yield %	1.77%		1.80%
Weighted Average Remaining Maturity/Duration	2 years 9 months 10 days		2 years 11 months 8 days
Recurring | GNMA permanent securities
Assets:
Outstanding Face Amount	$ 33,472		$ 33,745
Fair Value	34,113		34,742
Recurring | GNMA permanent securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	33,472		33,745
Amortized Cost Basis	33,757		34,386
Fair Value	$ 34,113		$ 34,742
Fair Value Assumptions:
Weighted average yield %	3.62%		3.62%
Weighted Average Remaining Maturity/Duration	5 years 6 months		5 years 7 months 27 days
Recurring | Mortgage loan receivables held for investment, net, at amortized cost
Assets:
Provision for loan losses	$ (7,000)		$ (4,000)
Recurring | Mortgage loan receivables held for investment, net, at amortized cost | Discounted Cash Flow
Assets:
Outstanding Face Amount	3,550,544		3,300,709
Amortized Cost Basis	3,528,185		3,282,462
Fair Value	$ 3,532,369		$ 3,292,035
Fair Value Assumptions:
Weighted average yield %	7.34%		7.18%
Weighted Average Remaining Maturity/Duration	1 year 7 months 2 days		1 year 7 months 9 days
Recurring | Mortgage loan receivables held for sale | Internal model, third-party inputs
Assets:
Outstanding Face Amount	$ 274,090		$ 232,527
Amortized Cost Basis	273,636		230,180
Fair Value	$ 279,372		$ 236,428
Fair Value Assumptions:
Weighted average yield %	5.09%		4.88%
Weighted Average Remaining Maturity/Duration	9 years 8 months 15 days		8 years 2 months 1 day
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	$ 713,900		$ 594,140
Fair Value	92		888
Recurring | Nonhedge derivatives | Counterparty quotations
Assets:
Derivative asset face amount	713,900		594,140
Fair Value	$ 92		$ 888
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	3 months		2 months 26 days

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