INTEREST INCOME AND INTEREST EXPENSE - Components of Company's Interest Income and Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest income:
Residential Investment Securities	$ 779,588	$ 515,910
Residential mortgage loans	15,505	3,864
Commercial investment portfolio	72,457	64,345
Reverse repurchase agreements	11,937	3,608
Total interest income	879,487	587,727
Interest expense:
Repurchase agreements	331,374	173,090
Securitized debt of consolidated VIEs	15,652	14,850
Participation sold	0	153
Other	20,395	10,332
Total interest expense	367,421	198,425
Net interest income	$ 512,066	$ 389,302