SECURED FINANCING - Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Gross amounts -reverse repurchase agreements	$ 2,909,762	$ 1,250,000
Amounts offset - reverse repurchase agreement	(2,650,000)	(1,250,000)
Netted amounts -reverse repurchase	259,762	0	[1]
Gross amounts -repurchase agreement	78,410,655	78,946,343
Amounts offset -repurchase agreement	(2,650,000)	(1,250,000)
Repurchase agreements	$ 75,760,655	$ 77,696,343	[1]

[1] Derived from the audited consolidated financial statements at December 31, 2017.