FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Assets:
Fair Value	$ 2,241,525,000	$ 2,034,719,000
Liabilities:
Fair Value	4,313,165,000	4,198,379,000
Repurchase agreements - short term
Liabilities:
Outstanding Face Amount	793,933,000	1,331,603,000
Fair Value	793,933,000	1,331,603,000
Repurchase agreements - short term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	793,933,000	1,331,603,000
Amortized Cost Basis	793,933,000	1,331,603,000
Fair Value	$ 793,933,000	$ 1,331,603,000
Fair Value Assumptions:
Weighted average yield %	1.83%	1.32%
Weighted Average Remaining Maturity/Duration	3 months 25 days	2 months 23 days
Repurchase agreements - long term
Liabilities:
Outstanding Face Amount	$ 397,393,000	$ 100,062,000
Fair Value	397,393,000	100,062,000
Repurchase agreements - long term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	397,393,000	100,062,000
Amortized Cost Basis	397,393,000	100,062,000
Fair Value	$ 397,393,000	$ 100,062,000
Fair Value Assumptions:
Weighted average yield %	1.30%	1.89%
Weighted Average Remaining Maturity/Duration	1 year 1 month 13 days	1 year 7 months 2 days
Borrowings under credit agreement
Liabilities:
Outstanding Face Amount	$ 20,400,000	$ 11,000,000
Fair Value	20,400,000	11,000,000
Borrowings under credit agreement | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	20,400,000	11,000,000
Amortized Cost Basis	20,400,000	11,000,000
Fair Value	$ 20,400,000	$ 11,000,000
Fair Value Assumptions:
Weighted average yield %	2.95%	2.91%
Weighted Average Remaining Maturity/Duration	3 months 25 days	1 year 25 days
Borrowings under credit and security agreement
Liabilities:
Outstanding Face Amount		$ 46,750,000
Fair Value		46,750,000
Borrowings under credit and security agreement | Discounted Cash Flow
Liabilities:
Outstanding Face Amount		46,750,000
Amortized Cost Basis		46,750,000
Fair Value		$ 46,750,000
Fair Value Assumptions:
Weighted average yield %		2.01%
Weighted Average Remaining Maturity/Duration		1 year 9 months 7 days
Revolving credit facility
Liabilities:
Outstanding Face Amount	$ 50,000,000	$ 25,000,000
Fair Value	50,000,000	25,000,000
Revolving credit facility | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	50,000,000	25,000,000
Amortized Cost Basis	50,000,000	25,000,000
Fair Value	$ 50,000,000	$ 25,000,000
Fair Value Assumptions:
Weighted average yield %	3.69%	3.66%
Weighted Average Remaining Maturity/Duration	1 year 4 months 13 days	2 years 1 month 13 days
Mortgage loan financing
Liabilities:
Outstanding Face Amount	$ 562,035,000	$ 442,753,000
Fair Value	590,265,000	455,846,000
Mortgage loan financing | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	562,035,000	442,753,000
Amortized Cost Basis	555,786,000	447,410,000
Fair Value	$ 590,265,000	$ 455,846,000
Fair Value Assumptions:
Weighted average yield %	4.87%	4.85%
Weighted Average Remaining Maturity/Duration	7 years 11 months 15 days	8 years 5 months 19 days
Impairment on security positions		$ 0
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	$ 1,786,000,000	1,611,000,000
Fair Value	1,857,923,000	1,616,373,000
Borrowings from the FHLB | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	1,786,000,000	1,611,000,000
Amortized Cost Basis	1,786,000,000	1,611,000,000
Fair Value	$ 1,857,923,000	$ 1,616,373,000
Fair Value Assumptions:
Weighted average yield %	0.83%	0.79%
Weighted Average Remaining Maturity/Duration	1 year 7 months 13 days	2 years 18 days
Senior unsecured notes
Liabilities:
Outstanding Face Amount	$ 619,555,000	$ 619,555,000
Fair Value	603,251,000	611,745,000
Senior unsecured notes | Broker quotations, pricing services
Liabilities:
Outstanding Face Amount	619,555,000	619,555,000
Amortized Cost Basis	619,555,000	619,555,000
Fair Value	$ 603,251,000	$ 611,745,000
Fair Value Assumptions:
Weighted average yield %	6.65%	6.65%
Weighted Average Remaining Maturity/Duration	3 years 10 months 9 days	4 years 7 months 9 days
Repurchase agreement liabilities
Fair Value Assumptions:
Period of short interest rate reset risk		30 days
Impairment on security positions		$ 0
Mortgage loan receivables held for investment, at amortized cost
Assets:
Outstanding Face Amount	$ 1,811,216,000	1,536,923,000
Fair Value	1,820,999,000	1,540,388,000
Mortgage loan receivables held for investment, at amortized cost | Discounted Cash Flow
Assets:
Outstanding Face Amount	1,811,216,000	1,536,923,000
Amortized Cost Basis	1,794,812,000	1,521,053,000
Fair Value	$ 1,820,999,000	$ 1,540,388,000
Fair Value Assumptions:
Weighted average yield %	7.61%	7.33%
Weighted Average Remaining Maturity/Duration	1 year 6 months 29 days	1 year 11 months 15 days
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	$ 333,488,000	$ 417,955,000
Fair Value	342,611,000	421,991,000
Mortgage loan receivables held for sale | Discounted Cash Flow
Assets:
Outstanding Face Amount	333,488,000	417,955,000
Amortized Cost Basis	333,531,000	417,955,000
Fair Value	$ 342,611,000	$ 421,991,000
Fair Value Assumptions:
Weighted average yield %	4.15%	4.31%
Weighted Average Remaining Maturity/Duration	7 years 7 months 6 days	9 years 8 months 19 days
FHLB stock
Assets:
Outstanding Face Amount	$ 77,915,000	$ 72,340,000
Fair Value	77,915,000	72,340,000
FHLB stock | Put Value
Assets:
Outstanding Face Amount	77,915,000	72,340,000
Amortized Cost Basis	77,915,000	72,340,000
Fair Value	$ 77,915,000	$ 72,340,000
Fair Value Assumptions:
Weighted average yield %	3.50%	3.50%
Mortgage loan receivables
Fair Value Assumptions:
Period of short interest rate reset risk		30 days
Recurring
Assets:
Fair Value	$ 2,415,681,000	$ 2,815,989,000
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	130,051,000	1,428,700,000
Fair Value	21,942,000	13,446,000
Recurring | Nonhedge derivatives | Counterparty quotations
Liabilities:
Derivative liability face amount	130,051,000	1,428,700,000
Fair Value	$ 21,942,000	$ 13,446,000
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	11 months 26 days	1 year 4 months 27 days
Recurring | CMBS
Assets:
Outstanding Face Amount	$ 1,936,316,000	$ 2,247,565,000
Fair Value	1,985,195,000	2,305,409,000
Recurring | CMBS | Internal model, third-party inputs
Assets:
Outstanding Face Amount	1,936,316,000	2,247,565,000
Amortized Cost Basis	1,960,371,000	2,277,995,000
Fair Value	$ 1,985,195,000	$ 2,305,409,000
Fair Value Assumptions:
Weighted average yield %	2.63%	2.60%
Weighted Average Remaining Maturity/Duration	3 years 4 months 6 days	4 years 2 months 23 days
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	$ 7,246,777,000	$ 7,239,503,000
Fair Value	356,928,000	378,335,000
Recurring | CMBS interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	7,246,777,000	7,239,503,000
Amortized Cost Basis	356,204,000	376,085,000
Fair Value	$ 356,928,000	$ 378,335,000
Fair Value Assumptions:
Weighted average yield %	3.74%	4.88%
Weighted Average Remaining Maturity/Duration	3 years 5 months 15 days	3 years 5 months 12 days
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	$ 662,161,000	$ 1,400,141,000
Fair Value	28,603,000	66,642,000
Recurring | GNMA interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	662,161,000	1,400,141,000
Amortized Cost Basis	30,109,000	67,543,000
Fair Value	$ 28,603,000	$ 66,642,000
Fair Value Assumptions:
Weighted average yield %	4.27%	5.90%
Weighted Average Remaining Maturity/Duration	5 years 5 months 23 days	4 years 6 months
Recurring | GN construction securities
Assets:
Outstanding Face Amount	$ 26,730,000	$ 27,538,000
Fair Value	27,849,000	28,406,000
Recurring | GN construction securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	26,730,000	27,538,000
Amortized Cost Basis	27,223,000	28,178,000
Fair Value	$ 27,849,000	$ 28,406,000
Fair Value Assumptions:
Weighted average yield %	3.86%	3.56%
Weighted Average Remaining Maturity/Duration	9 years 5 months 15 days	9 years 5 months 1 day
Recurring | GN permanent securities
Assets:
Outstanding Face Amount	$ 16,435,000	$ 36,232,000
Fair Value	16,807,000	36,773,000
Recurring | GN permanent securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	16,435,000	36,232,000
Amortized Cost Basis	16,950,000	36,515,000
Fair Value	$ 16,807,000	$ 36,773,000
Fair Value Assumptions:
Weighted average yield %	4.00%	4.94%
Weighted Average Remaining Maturity/Duration	6 years 1 month 2 days	1 year 3 months 25 days
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	$ 10,009,000	$ 125,050,000
Fair Value	299,000	424,000
Recurring | Nonhedge derivatives | Counterparty quotations
Assets:
Derivative asset face amount	10,009,000	125,050,000
Fair Value	$ 299,000	$ 424,000
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	5 years 7 months 2 days	3 years 5 months 12 days