LOANS - Summary of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Mortgage Loans on Real Estate [Line Items]
Net interest income	$ 315,623	$ 353,613	$ 1,161,803	$ 1,058,060
Net gains (losses) on disposal of investments			(376,943)	(11,833)
Total included in net income (loss)	385,578	367,547	2,309,297	822,682
Residential mortgage loans
Mortgage Loans on Real Estate [Line Items]
Net interest income	16,423	8,226	45,702	18,935
Net gains (losses) on disposal of investments	(2,975)	(2,093)	(7,924)	(3,407)
Net unrealized gains (losses) on instruments measured at fair value through earnings	(3,633)	(725)	(14,802)	5,400
Total included in net income (loss)	$ 9,815	$ 5,408	$ 22,976	$ 20,928