DERIVATIVES - Gain (Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Other income (loss), net	$ (113,558)	$ (2,190)	$ 5,774
Gain (loss) on settlement of investments, net	54,867	(39,214)	(27,491)
Total gains (losses)	(58,691)	(41,404)	(21,717)
Gain (loss) on settlement of mortgage loan origination derivative instruments	1,234
Interest Rate Caps
Derivative [Line Items]
Other income (loss), net	431	323	688
Gain (loss) on settlement of investments, net	(603)	(1,911)	(4,754)
Interest Rate Swaps
Derivative [Line Items]
Other income (loss), net	(108,098)	(720)	5,500
Gain (loss) on settlement of investments, net	65,823	6,921	(4,810)
TBAs
Derivative [Line Items]
Other income (loss), net	(5,631)	(1,793)	(414)
Gain (loss) on settlement of investments, net	(10,353)	(44,224)	(17,927)
Interest Rate Lock Commitments
Derivative [Line Items]
Other income (loss), net	23	0	0
Forward Loan Sale Commitments
Derivative [Line Items]
Other income (loss), net	$ (283)	$ 0	$ 0