DERIVATIVE INSTRUMENTS - Schedule of Derivatives Outstanding (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Notional	$ 1,228,700,000	$ 1,553,750,000
Fair value, Asset	299,000	423,000
Fair value, Liability	21,942,000	13,445,000
1MO LIBOR
Derivative [Line Items]
Notional		71,250,000
Fair value, Asset		0
Fair value, Liability		$ 0
Remaining Maturity		7 months 27 days
5-year Swap
Derivative [Line Items]
Notional	591,800,000	$ 496,200,000
Fair value, Asset	8,000	108,000
Fair value, Liability	$ 6,575,000	$ 28,000
Remaining Maturity	3 months	3 months
Term of derivative contract	5 years	5 years
10-year Swap
Derivative [Line Items]
Notional	$ 513,400,000	$ 842,800,000
Fair value, Asset	2,000	104,000
Fair value, Liability	$ 9,976,000	$ 8,258,000
Remaining Maturity	3 months	3 months
Term of derivative contract	10 years	10 years
Futures
Derivative [Line Items]
Notional	$ 1,105,200,000	$ 1,339,000,000
Fair value, Asset	10,000	212,000
Fair value, Liability	16,551,000	8,286,000
Swaps 3MO LIB
Derivative [Line Items]
Notional	80,000,000	100,000,000
Fair value, Asset	0	0
Fair value, Liability	$ 5,026,000	$ 4,505,000
Remaining Maturity	3 years 2 months 26 days	3 years 2 months 4 days
CMBX
Derivative [Line Items]
Notional	$ 33,500,000	$ 10,000,000
Fair value, Asset	0	211,000
Fair value, Liability	$ 365,000	$ 0
Remaining Maturity	3 years 3 months 21 days	6 years 9 months 18 days
CDX
Derivative [Line Items]
Notional	$ 10,000,000	$ 33,500,000
Fair value, Asset	289,000	0
Fair value, Liability	$ 0	$ 654,000
Remaining Maturity	5 years 9 months 3 days	3 years 11 months 19 days
Credit Derivatives
Derivative [Line Items]
Notional	$ 43,500,000	$ 43,500,000
Fair value, Asset	289,000	211,000
Fair value, Liability	$ 365,000	$ 654,000