DERIVATIVE INSTRUMENTS - Schedule of Derivatives Outstanding (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 1,037,900,000	$ 1,242,900,000
Fair value, Asset	215,000	2,821,000
Fair value, Liability	12,743,000	5,504,000
5-year Swap
Derivative [Line Items]
Notional	542,900,000	670,100,000
Fair value, Asset	9,000	2,122,000
Fair value, Liability	$ 4,416,000	$ 0
Remaining Maturity	3 months	3 months
Term of derivative contract	5 years	5 years
10-year Swap
Derivative [Line Items]
Notional	$ 399,700,000	$ 477,900,000
Fair value, Asset	0	463,000
Fair value, Liability	$ 3,018,000	$ 1,451,000
Remaining Maturity	3 months	3 months
Term of derivative contract	10 years	10 years
5-year U.S. Treasury Note
Derivative [Line Items]
Notional	$ 700,000	$ 800,000
Fair value, Asset	0	3,000
Fair value, Liability	$ 1,000	$ 0
Remaining Maturity	3 months	3 months
10-year U.S. Treasury Note Ultra
Derivative [Line Items]
Notional	$ 1,100,000
Fair value, Asset	3,000
Fair value, Liability	$ 0
Remaining Maturity	3 months
10-year U.S. Treasury Note
Derivative [Line Items]
Notional		$ 600,000
Fair value, Asset		3,000
Fair value, Liability		$ 0
Remaining Maturity		3 months
Futures
Derivative [Line Items]
Notional	$ 944,400,000	$ 1,149,400,000
Fair value, Asset	12,000	2,591,000
Fair value, Liability	7,435,000	1,451,000
3MO LIBOR
Derivative [Line Items]
Notional	50,000,000	50,000,000
Fair value, Asset	0	0
Fair value, Liability	$ 4,980,000	$ 3,686,000
Remaining Maturity	4 years 5 months 19 days	4 years 8 months 19 days
CMBX
Derivative [Line Items]
Notional	$ 10,000,000	$ 10,000,000
Fair value, Asset	203,000	230,000
Fair value, Liability	$ 0	$ 0
Remaining Maturity	5 years 5 months 12 days	5 years 7 months 2 days
CDX
Derivative [Line Items]
Notional	$ 33,500,000	$ 33,500,000
Fair value, Asset	0	0
Fair value, Liability	$ 328,000	$ 367,000
Remaining Maturity	2 years 8 months 12 days	2 years 11 months 1 day
Credit Derivatives
Derivative [Line Items]
Notional	$ 43,500,000	$ 43,500,000
Fair value, Asset	203,000	230,000
Fair value, Liability	$ 328,000	$ 367,000