DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Dec. 31, 2017		Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Interest rate swaps, at fair value	$ 30,272,000	[1]	$ 68,194,000
Other derivatives, at fair value	283,613,000	[1]	171,266,000
Derivative assets	313,885,000		239,460,000
Interest rate swaps, at fair value	569,129,000	[1]	1,443,765,000
Other derivatives, at fair value	38,725,000	[1]	86,437,000
Derivative liabilities	607,854,000		1,530,202,000
Futures contracts
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	218,361,000		168,209,000
Derivative assets	218,361,000		168,209,000
Other derivatives, at fair value	12,285,000		24,912,000
Derivative liabilities	12,285,000		24,912,000
Purchase commitments
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	35,000		283,000
Derivative assets	35,000		283,000
Other derivatives, at fair value	157,000		553,000
Derivative liabilities	157,000		553,000
Interest rate swaptions
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	36,150,000		0
Derivative assets	36,150,000
TBA derivatives
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	29,067,000		2,774,000
Derivative assets	29,067,000		2,774,000
Other derivatives, at fair value	21,776,000		60,972,000
Derivative liabilities	21,776,000		60,972,000
Notional	15,578,000,000
Credit derivatives
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	4,507,000		$ 0
Derivative liabilities	4,507,000
Notional	$ 125,000,000.0

[1] As a result of a change to a clearing organization’s rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.