FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Values of Financial Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Repurchase agreements - short term
Liabilities:
Outstanding Face Amount	$ 1,331,603,000	$ 409,334,000
Fair Value	1,331,603,000	409,334,000
Repurchase agreements - short term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	1,331,603,000	409,334,000
Amortized Cost Basis	1,331,603,000	409,334,000
Fair Value	1,331,603,000	409,334,000
Fair Value Assumptions:
Weighted average yield %	1.32%	1.46%
Weighted Average Remaining Maturity/Duration	2 months 23 days	14 days
Repurchase agreements - long term
Liabilities:
Outstanding Face Amount	100,062,000	200,501,000
Fair Value	100,062,000	200,501,000
Repurchase agreements - long term | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	100,062,000	200,501,000
Amortized Cost Basis	100,062,000	200,501,000
Fair Value	100,062,000	200,501,000
Fair Value Assumptions:
Weighted average yield %	1.89%	2.13%
Weighted Average Remaining Maturity/Duration	1 year 7 months 2 days	1 year 5 months 26 days
Borrowings under credit agreement
Liabilities:
Outstanding Face Amount	11,000,000
Fair Value	11,000,000
Borrowings under credit agreement | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	11,000,000
Amortized Cost Basis	11,000,000
Fair Value	11,000,000
Fair Value Assumptions:
Weighted average yield %	2.91%
Weighted Average Remaining Maturity/Duration	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	25,000,000
Fair Value	25,000,000
Revolving credit facility | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	25,000,000
Amortized Cost Basis	25,000,000
Fair Value	25,000,000
Fair Value Assumptions:
Weighted average yield %	3.66%
Weighted Average Remaining Maturity/Duration	2 years 1 month 13 days
Mortgage loan financing
Liabilities:
Outstanding Face Amount	442,753,000	287,246,000
Fair Value	455,846,000	278,129,000
Mortgage loan financing | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	442,753,000	287,246,000
Amortized Cost Basis	447,410,000	291,053,000
Fair Value	455,846,000	278,129,000
Fair Value Assumptions:
Weighted average yield %	4.85%	4.84%
Weighted Average Remaining Maturity/Duration	8 years 5 months 19 days	8 years 8 months 12 days
Impairment on security positions	0	0
Borrowings from the FHLB
Liabilities:
Outstanding Face Amount	1,611,000,000	989,000,000
Fair Value	1,616,373,000	987,896,000
Borrowings from the FHLB | Discounted Cash Flow
Liabilities:
Outstanding Face Amount	1,611,000,000	989,000,000
Amortized Cost Basis	1,611,000,000	989,000,000
Fair Value	1,616,373,000	987,896,000
Fair Value Assumptions:
Weighted average yield %	0.79%	0.57%
Weighted Average Remaining Maturity/Duration	2 years 18 days	1 year 7 months 6 days
Senior unsecured notes
Liabilities:
Outstanding Face Amount	619,555,000	325,000,000
Fair Value	611,745,000	341,250,000
Senior unsecured notes | Broker quotations, pricing services
Liabilities:
Outstanding Face Amount	619,555,000	325,000,000
Amortized Cost Basis	619,555,000	325,000,000
Fair Value	611,745,000	341,250,000
Fair Value Assumptions:
Weighted average yield %	6.65%	7.38%
Weighted Average Remaining Maturity/Duration	4 years 7 months 9 days	3 years 9 months
Repurchase agreement liabilities
Fair Value Assumptions:
Period of short interest rate reset risk	30 days	30 days
Impairment on security positions	0	0
Mortgage loan receivables held for investment, at amortized cost
Assets:
Outstanding Face Amount	1,536,923,000	549,574,000
Fair Value	1,540,388,000	541,578,000
Mortgage loan receivables held for investment, at amortized cost | Discounted Cash Flow
Assets:
Outstanding Face Amount	1,536,923,000	549,574,000
Amortized Cost Basis	1,521,053,000	539,078,000
Fair Value	1,540,388,000	541,578,000
Fair Value Assumptions:
Weighted average yield %	7.33%	9.76%
Weighted Average Remaining Maturity/Duration	1 year 11 months 15 days	2 years 1 month 20 days
Mortgage loan receivables held for sale
Assets:
Outstanding Face Amount	417,955,000	440,775,000
Fair Value	421,991,000	455,804,000
Mortgage loan receivables held for sale | Discounted Cash Flow
Assets:
Outstanding Face Amount	417,955,000	440,775,000
Amortized Cost Basis	417,955,000	440,490,000
Fair Value	421,991,000	455,804,000
Fair Value Assumptions:
Weighted average yield %	4.31%	5.47%
Weighted Average Remaining Maturity/Duration	9 years 8 months 19 days	9 years 7 months 13 days
FHLB stock
Assets:
Outstanding Face Amount	72,340,000	49,450,000
Fair Value	72,340,000	49,450,000
FHLB stock | Put Value
Assets:
Outstanding Face Amount	72,340,000	49,450,000
Amortized Cost Basis	72,340,000	49,450,000
Fair Value	72,340,000	49,450,000
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	30 days
Recurring | Nonhedge derivatives
Liabilities:
Derivative liability face amount	1,428,700,000	154,500,000
Fair Value	13,446,000	7,031,000
Recurring | Nonhedge derivatives | Counterparty quotations
Liabilities:
Derivative liability face amount	1,428,700,000	154,500,000
Fair Value	13,446,000	7,031,000
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	1 year 4 months 27 days	4 years 6 months 18 days
Recurring | CMBS
Assets:
Outstanding Face Amount	2,247,565,000	1,160,741,000
Fair Value	2,305,409,000	1,164,936,000
Recurring | CMBS | Internal model, third-party inputs
Assets:
Outstanding Face Amount	2,247,565,000
Amortized Cost Basis	2,277,995,000
Fair Value	2,305,409,000
Fair Value Assumptions:
Weighted average yield %	2.60%
Weighted Average Remaining Maturity/Duration	4 years 2 months 23 days
Recurring | CMBS | Broker quotations, pricing services
Assets:
Outstanding Face Amount		1,160,741,000
Amortized Cost Basis		1,156,230,000
Fair Value		1,164,936,000
Fair Value Assumptions:
Weighted average yield %		4.08%
Weighted Average Remaining Maturity/Duration		4 years 10 months 17 days
Recurring | CMBS interest-only
Assets:
Outstanding Face Amount	7,239,503,000	5,702,862,000
Fair Value	378,335,000	258,058,000
Recurring | CMBS interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	7,239,503,000
Amortized Cost Basis	376,085,000
Fair Value	378,335,000
Fair Value Assumptions:
Weighted average yield %	4.88%
Weighted Average Remaining Maturity/Duration	3 years 5 months 12 days
Recurring | CMBS interest-only | Broker quotations, pricing services
Assets:
Outstanding Face Amount		5,702,862,000
Amortized Cost Basis		256,869,000
Fair Value		258,058,000
Fair Value Assumptions:
Weighted average yield %		4.19%
Weighted Average Remaining Maturity/Duration		3 years 4 months 17 days
Recurring | GNMA interest-only
Assets:
Outstanding Face Amount	1,400,141,000	1,848,270,000
Fair Value	66,642,000	99,877,000
Recurring | GNMA interest-only | Internal model, third-party inputs
Assets:
Outstanding Face Amount	1,400,141,000
Amortized Cost Basis	67,543,000
Fair Value	66,642,000
Fair Value Assumptions:
Weighted average yield %	5.90%
Weighted Average Remaining Maturity/Duration	4 years 6 months
Recurring | GNMA interest-only | Broker quotations, pricing services
Assets:
Outstanding Face Amount		1,848,270,000
Amortized Cost Basis		103,136,000
Fair Value		99,877,000
Fair Value Assumptions:
Weighted average yield %		5.32%
Weighted Average Remaining Maturity/Duration		2 years 1 month 13 days
Recurring | FHLMC interest-only
Assets:
Outstanding Face Amount		219,677,000
Fair Value		8,152,000
Recurring | FHLMC interest-only | Broker quotations, pricing services
Assets:
Outstanding Face Amount		219,677,000
Amortized Cost Basis		7,904,000
Fair Value		8,152,000
Fair Value Assumptions:
Weighted average yield %		5.21%
Weighted Average Remaining Maturity/Duration		3 years 14 days
Recurring | GN construction securities
Assets:
Outstanding Face Amount	27,538,000	12,858,000
Fair Value	28,406,000	13,007,000
Recurring | GN construction securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	27,538,000
Amortized Cost Basis	28,178,000
Fair Value	28,406,000
Fair Value Assumptions:
Weighted average yield %	3.56%
Weighted Average Remaining Maturity/Duration	9 years 5 months 1 day
Recurring | GN construction securities | Broker quotations, pricing services
Assets:
Outstanding Face Amount		12,858,000
Amortized Cost Basis		13,261,000
Fair Value		13,007,000
Fair Value Assumptions:
Weighted average yield %		3.49%
Weighted Average Remaining Maturity/Duration		6 years 6 months 25 days
Recurring | GN permanent securities
Assets:
Outstanding Face Amount	36,232,000	108,310,000
Fair Value	36,773,000	113,216,000
Recurring | GN permanent securities | Internal model, third-party inputs
Assets:
Outstanding Face Amount	36,232,000
Amortized Cost Basis	36,515,000
Fair Value	36,773,000
Fair Value Assumptions:
Weighted average yield %	4.94%
Weighted Average Remaining Maturity/Duration	1 year 3 months 25 days
Recurring | GN permanent securities | Broker quotations, pricing services
Assets:
Outstanding Face Amount		108,310,000
Amortized Cost Basis		110,724,000
Fair Value		113,216,000
Fair Value Assumptions:
Weighted average yield %		4.64%
Weighted Average Remaining Maturity/Duration		3 years 3 months 7 days
Recurring | Nonhedge derivatives
Assets:
Derivative asset face amount	125,050,000	808,700,000
Fair Value	424,000	8,244,000
Recurring | Nonhedge derivatives | Counterparty quotations
Assets:
Derivative asset face amount	125,050,000	808,700,000
Fair Value	$ 424,000	$ 8,244,000
Fair Value Assumptions:
Weighted Average Remaining Maturity/Duration	3 years 5 months 12 days	6 months