INTEREST INCOME AND INTEREST EXPENSE - Components of Company's Interest Income and Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest income:
Residential Investment Securities	$ 662,750	$ 459,308	$ 1,442,338	$ 975,218
Residential mortgage loans	18,868	7,417	34,373	11,281
Commercial investment portfolio	79,343	68,153	151,800	132,498
Reverse repurchase agreements	15,845	2,548	27,782	6,156
Total interest income	776,806	537,426	1,656,293	1,125,153
Interest expense:
Repurchase agreements	400,475	197,151	731,849	370,241
Securitized debt of consolidated VIEs	18,201	11,977	33,853	26,827
Participation sold	0	42	0	195
Other	24,016	13,111	44,411	23,443
Total interest expense	442,692	222,281	810,113	420,706
Net interest income	$ 334,114	$ 315,145	$ 846,180	$ 704,447