INVESTMENTS IN RESIDENTIAL MORTGAGE LOANS - Activities Related to the Carrying Value of Loans Held-for-sale (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans Held-for-sale, Reconciliation [Roll Forward]
Transfers of loans from held-for-investment	$ 23,080	$ 316,199	$ 0
Transfer of loans to real estate owned	(124,013)	(81,940)
Loans Held-for-sale
Loans Held-for-sale, Reconciliation [Roll Forward]
Beginning balance, loans held-for-sale	696,665	776,681
Purchases	5,135,700	1,196,018
Transfers of loans from held-for-investment	23,080	316,199
Sales	(3,901,161)	(1,274,707)
Transfer of loans to other assets	(17,487)	(158,807)
Transfer of loans to real estate owned	(71,756)	(56,001)
Proceeds from repayments	(125,987)	(91,339)
Valuation (provision) reversal on loans	(13,520)	(11,379)
Ending balance, loans held-for-sale	1,725,534	696,665	$ 776,681
Provision for loans held-for-sale	$ 30,100	$ 30,200