RESIDENTIAL MORTGAGE LOANS - Schedule of Residential Mortgage Loans (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Movement in Mortgage Loans on Real Estate [Roll Forward]
Fair value, beginning of period	[1]	$ 342,289
Principal repayments		(349,220)		$ (654,117)
Fair value, end of period	[1]	1,438,322	[2]	342,289
Residential Mortgage
Movement in Mortgage Loans on Real Estate [Roll Forward]
Fair value, beginning of period		342,289		0
Purchases		1,218,491		165,213
Consolidation of VIEs		349,200		0
Sales		(278,305)		(134,203)
Principal repayments		(189,465)		(42,612)
Amortization of premiums		(1,523)		(942)
Change in fair value		(2,365)		(5,614)
Fair value, end of period		1,438,322		342,289
Residential Mortgage | Hatteras
Movement in Mortgage Loans on Real Estate [Roll Forward]
Obtained through Hatteras Acquisition		$ 0		$ 360,447

[1] Includes securitized residential mortgage loans of a consolidated VIE carried at fair value of $478.8 million and $165.9 million at December 31, 2017 and 2016, respectively

[2] As a result of a change to a clearing organization’s rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date will not be adjusted.