INVESTMENTS IN REAL ESTATE AND OTHER SECURITIES - Narrative (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
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairment (OTTI) on securities	$ 1,598	$ 1,509	$ 5,115	$ 2,112	$ 2,426	$ 1,765	$ 2,819	$ 3,254	$ 10,334	$ 10,264	$ 5,788
Real estate securities acquired during the period with credit quality deterioration, face amount									3,148,300	2,510,300
Real estate securities acquired with credit quality deterioration, expected cash flows									2,699,700	2,490,700
Real estate securities acquired with credit quality deterioration, fair value									1,836,100	$ 1,538,500
Agency
Schedule of Available-for-sale Securities [Line Items]
Face amount of securities sold	1,000,000								1,000,000
Face amount of securities purchased, unsettled	1,100,000								1,100,000
Sale price of securities sold	1,000,000								1,000,000
Purchase of real estate securities, unsettled	1,100,000								1,100,000
Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Face amount of securities sold	45,000								45,000
Purchase of real estate securities, unsettled	$ 41,100								$ 41,100