DERIVATIVE INSTRUMENTS - Schedule of Derivatives Outstanding (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 1,276,511,000	$ 1,242,900,000
Fair value, Asset	254,000	2,821,000
Fair value, Liability	9,368,000	5,504,000
5-year Swap
Derivative [Line Items]
Notional	643,200,000	670,100,000
Fair value, Asset	99,000	2,122,000
Fair value, Liability	$ 1,607,000	$ 0
Remaining Maturity	3 months	3 months
Term of derivative contract	5 years	5 years
10-year Swap
Derivative [Line Items]
Notional	$ 517,500,000	$ 477,900,000
Fair value, Asset	58,000	463,000
Fair value, Liability	$ 2,816,000	$ 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	$ 21,200,000	$ 800,000
Fair value, Asset	5,000	3,000
Fair value, Liability	$ 2,000	$ 0
Remaining Maturity	3 months	3 months
10-year U.S. Treasury Note Ultra
Derivative [Line Items]
Notional	$ 1,100,000
Fair value, Asset	2,000
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	$ 1,183,000,000	$ 1,149,400,000
Fair value, Asset	164,000	2,591,000
Fair value, Liability	4,425,000	1,451,000
3 Month LIBOR
Derivative [Line Items]
Notional	50,000,000	50,000,000
Fair value, Asset	0	0
Fair value, Liability	$ 4,474,000	$ 3,686,000
Remaining Maturity	3 years 11 months 19 days	4 years 8 months 19 days
CMBX
Derivative [Line Items]
Notional	$ 10,000,000	$ 10,000,000
Fair value, Asset	79,000	230,000
Fair value, Liability	$ 0	$ 0
Remaining Maturity	5 years 3 months 25 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	$ 469,000	$ 367,000
Remaining Maturity	2 years 2 months 4 days	2 years 11 months 1 day
S&P 500 Put Option
Derivative [Line Items]
Notional	$ 11,000
Fair value, Asset	11,000
Fair value, Liability	$ 0
Remaining Maturity	2 months 15 days
Credit Derivatives
Derivative [Line Items]
Notional	$ 43,511,000	$ 43,500,000
Fair value, Asset	90,000	230,000
Fair value, Liability	$ 469,000	$ 367,000