DERIVATIVE INSTRUMENTS - Schedule of Derivatives Outstanding (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016
Derivative [Line Items]
Notional	$ 648,300,000		$ 947,400,000
Fair value, Asset	888,000	[1]	5,018,000
Fair value, Liability	2,606,000	[1]	3,446,000
5-year Swap
Derivative [Line Items]
Notional	304,300,000		602,200,000
Fair value, Asset	656,000		3,210,000
Fair value, Liability	$ 0		$ 2,000
Remaining Maturity	3 months		3 months
10-year Swap
Derivative [Line Items]
Notional	$ 248,100,000		$ 226,700,000
Fair value, Asset	133,000		1,674,000
Fair value, Liability	$ 153,000		$ 266,000
Remaining Maturity	3 months		3 months
5-year U.S. Treasury Note
Derivative [Line Items]
Notional	$ 11,400,000		$ 21,800,000
Fair value, Asset	47,000		93,000
Fair value, Liability	$ 0		$ 0
Remaining Maturity	3 months		3 months
10-year U.S. Treasury Note
Derivative [Line Items]
Notional			$ 3,200,000
Fair value, Asset			38,000
Fair value, Liability			$ 0
Remaining Maturity			3 months
Variation Margin
Derivative [Line Items]
Notional	$ 0
Fair value, Asset	0
Fair value, Liability	911,000
Futures
Derivative [Line Items]
Notional	563,800,000		$ 853,900,000
Fair value, Asset	836,000		5,015,000
Fair value, Liability	1,064,000		268,000
3 Month LIBOR
Derivative [Line Items]
Notional	50,000,000		50,000,000
Fair value, Asset	0		0
Fair value, Liability	$ 1,542,000		$ 2,697,000
Remaining Maturity	2 years 8 months 4 days		3 years 8 months 19 days
VIX
Derivative [Line Items]
Notional	$ 34,500,000
Fair value, Asset	52,000
Fair value, Liability	$ 0
Remaining Maturity	1 month 13 days
CMBX
Derivative [Line Items]
Notional			$ 10,000,000
Fair value, Asset			3,000
Fair value, Liability			$ 0
Remaining Maturity			5 years 29 days
CDX
Derivative [Line Items]
Notional			$ 33,500,000
Fair value, Asset			0
Fair value, Liability			$ 481,000
Remaining Maturity			1 year 11 months 19 days
Credit Derivatives
Derivative [Line Items]
Notional	$ 34,500,000		$ 43,500,000
Fair value, Asset	52,000		3,000
Fair value, Liability	$ 0		$ 481,000

[1] Includes amounts relating to consolidated variable interest entities. See Note 3.