SECURED FINANCING - Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Gross amounts -reverse repurchase agreements	$ 1,484,704	$ 1,250,000
Amounts offset - reverse repurchase agreement	(250,000)	(1,250,000)
Netted amounts -reverse repurchase	1,234,704	0
Gross amounts -repurchase agreement	79,323,026	78,946,343
Amounts offset -repurchase agreement	(250,000)	(1,250,000)
Repurchase agreements	$ 79,073,026	$ 77,696,343