DERIVATIVE INSTRUMENTS - Schedule of Derivatives Outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Notional	$ 947,400,000	$ 1,242,900,000
Fair value, Asset	5,018,000	2,821,000
Fair value, Liability	3,446,000	5,504,000
5-year Swap
Derivative [Line Items]
Notional	602,200,000	670,100,000
Fair value, Asset	3,210,000	2,122,000
Fair value, Liability	$ 2,000	$ 0
Remaining Maturity	7 days	7 days
Term of derivative contract	5 years	5 years
10-year Swap
Derivative [Line Items]
Notional	$ 226,700,000	$ 477,900,000
Fair value, Asset	1,674,000	463,000
Fair value, Liability	$ 266,000	$ 1,451,000
Remaining Maturity	7 days	7 days
Term of derivative contract	10 years	10 years
5-year U.S. Treasury Note
Derivative [Line Items]
Notional	$ 21,800,000	$ 800,000
Fair value, Asset	93,000	3,000
Fair value, Liability	$ 0	$ 0
Remaining Maturity	7 days	7 days
Term of derivative contract	5 years	5 years
10-year U.S. Treasury Note Ultra
Derivative [Line Items]
Notional	$ 3,200,000
Fair value, Asset	38,000
Fair value, Liability	$ 0
Remaining Maturity	7 days
Term of derivative contract	10 years
10-year U.S. Treasury Note
Derivative [Line Items]
Notional		$ 600,000
Fair value, Asset		3,000
Fair value, Liability		$ 0
Remaining Maturity		7 days
Term of derivative contract		10 years
Futures
Derivative [Line Items]
Notional	$ 853,900,000	$ 1,149,400,000
Fair value, Asset	5,015,000	2,591,000
Fair value, Liability	268,000	1,451,000
3 Month LIBOR
Derivative [Line Items]
Notional	50,000,000	50,000,000
Fair value, Asset	0	0
Fair value, Liability	$ 2,697,000	$ 3,686,000
Remaining Maturity	3 years 8 months 19 days	4 years 8 months 19 days
CMBX
Derivative [Line Items]
Notional	$ 10,000,000	$ 10,000,000
Fair value, Asset	3,000	230,000
Fair value, Liability	$ 0	$ 0
Remaining Maturity	5 years 29 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	$ 481,000	$ 367,000
Remaining Maturity	1 year 11 months 19 days	2 years 11 months 1 day
Credit Derivatives
Derivative [Line Items]
Notional	$ 43,500,000	$ 43,500,000
Fair value, Asset	3,000	230,000
Fair value, Liability	$ 481,000	$ 367,000