DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Interest rate swaps, at fair value	$ 48,114,000		$ 30,272,000
Derivative assets	200,503,000		313,885,000
Interest rate swaps, at fair value	420,365,000		569,129,000
Derivative liabilities	889,750,000		607,854,000
Futures contracts
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	0		218,361,000
Derivative assets			218,361,000
Other derivatives, at fair value	462,309,000		12,285,000
Derivative liabilities		$ 462,309,000	12,285,000
Purchase commitments
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	844,000		35,000
Derivative assets		844,000	35,000
Other derivatives, at fair value	33,000		157,000
Derivative liabilities		33,000	157,000
Interest rate swaptions
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	7,216,000		36,150,000
Derivative assets		7,216,000	36,150,000
TBA derivatives
Derivatives, Fair Value [Line Items]
Other derivatives, at fair value	141,688,000		29,067,000
Derivative assets		141,688,000	29,067,000
Other derivatives, at fair value	0		21,776,000
Derivative liabilities			21,776,000
Notional			15,578,000,000
Credit derivatives
Derivatives, Fair Value [Line Items]
Credit derivatives	2,641,000		0
Derivative assets		2,641,000
Other derivatives, at fair value	7,043,000		4,507,000
Derivative liabilities		$ 7,043,000	4,507,000
Notional amount of derivative assets	30,000,000.0
Notional	$ 451,000,000		$ 125,000,000