DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Derivative Assets:
Interest rate swaps	$ 97,002,000	$ 30,272,000
Derivative assets	404,841,000	313,885,000
Derivative Liabilities:
Interest rate swaps, at fair value	311,729,000	569,129,000
Derivative liabilities	379,794,000	607,854,000
Futures contracts
Derivative Assets:
Derivative assets	221,516,000	218,361,000
Derivative assets	221,516,000	218,361,000
Derivative Liabilities:
Other derivatives, at fair value	482,000	12,285,000
Derivative liabilities	482,000	12,285,000
Purchase commitments
Derivative Assets:
Derivative assets	189,000	35,000
Derivative assets	189,000	35,000
Derivative Liabilities:
Other derivatives, at fair value	797,000	157,000
Derivative liabilities	797,000	157,000
Interest rate swaptions
Derivative Assets:
Derivative assets	65,356,000	36,150,000
Derivative assets	65,356,000	36,150,000
TBA derivatives
Derivative Assets:
Derivative assets	13,535,000	29,067,000
Derivative assets	13,535,000	29,067,000
Derivative Liabilities:
Other derivatives, at fair value	64,900,000	21,776,000
Derivative liabilities	64,900,000	21,776,000
Notional		15,578,000,000
Credit derivatives
Derivative Assets:
Credit derivatives	7,243,000	0
Derivative assets	7,243,000
Derivative Liabilities:
Other derivatives, at fair value	1,886,000	4,507,000
Derivative liabilities	1,886,000	4,507,000
Notional amount of derivative assets	70,000,000
Notional	$ 466,000,000	$ 125,000,000