DERIVATIVE INSTRUMENTS - Effect of Other Derivative Contracts on the Consolidated Statements of Operations and Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Unrealized Gains (Losses) on Interest Rate Swaps	$ 512,918	$ 282,190	$ (124,869)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	261,418	230,582
U.S. Treasury futures
Derivative [Line Items]
Realized Gain (Loss)	20,459	(51,148)
Unrealized Gains (Losses) on Interest Rate Swaps	62,778	163,631
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	83,237	112,483
Purchase commitments
Derivative [Line Items]
Realized Gain (Loss)	0	0
Unrealized Gains (Losses) on Interest Rate Swaps	162	(123)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	162	(123)
TBA derivatives
Derivative [Line Items]
Realized Gain (Loss)	154,575	164,008
Unrealized Gains (Losses) on Interest Rate Swaps	65,490	(50,636)
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	220,065	113,372
Interest rate swaptions
Derivative [Line Items]
Realized Gain (Loss)	(935)	4,850
Unrealized Gains (Losses) on Interest Rate Swaps	(42,660)	0
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	(43,595)	$ 4,850
Credit derivatives
Derivative [Line Items]
Realized Gain (Loss)	1,521
Unrealized Gains (Losses) on Interest Rate Swaps	28
Amount of Gain/(Loss) Recognized in Net Gains (Losses) on Trading Assets	$ 1,549