INTEREST INCOME AND INTEREST EXPENSE - Components of Company's Interest Income and Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest income:
Residential Securities	$ 680,037	$ 540,436	$ 2,122,375	$ 1,515,654
Residential mortgage loans	21,184	8,509	55,557	19,790
Commercial investment portfolio	97,531	67,790	249,331	200,288
U.S. Treasury securities	160	0	160	0
Reverse repurchase agreements	17,684	5,815	45,466	11,971
Total interest income	816,596	622,550	2,472,889	1,747,703
Interest expense:
Repurchase agreements	445,535	237,669	1,177,384	607,910
Debt issued by securitization vehicles	29,391	16,072	63,244	42,899
Participation sold	0	0	0	195
Other	26,047	15,196	70,458	38,639
Total interest expense	500,973	268,937	1,311,086	689,643
Net interest income	$ 315,623	$ 353,613	$ 1,161,803	$ 1,058,060