INTEREST INCOME AND INTEREST EXPENSE - Components of Company's Interest Income and Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest income:
Residential Investment Securities									$ 2,170,041	$ 1,944,457	$ 1,963,629
Residential mortgage loans									30,540	4,147	0
Commercial investment portfolio									273,884	252,436	203,804
Reverse repurchase agreements									18,661	9,911	3,264
Total interest income	$ 745,423	$ 622,550	$ 537,426	$ 587,727	$ 807,022	$ 558,668	$ 457,118	$ 388,143	2,493,126	2,210,951	2,170,697
Interest expense:
Repurchase agreements									891,819	585,826	420,325
Convertible Senior Notes									0	0	29,740
Securitized debt of consolidated VIEs									60,304	44,392	20,065
Participation sold									195	627	639
Other									56,036	26,907	827
Total interest expense	318,711	268,937	222,281	198,425	183,396	174,154	152,755	147,447	1,008,354	657,752	471,596
Net interest income	$ 426,712	$ 353,613	$ 315,145	$ 389,302	$ 623,626	$ 384,514	$ 304,363	$ 240,696	$ 1,484,772	$ 1,553,199	$ 1,699,101