SECURED FINANCING - Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Gross amounts -reverse repurchase agreements	$ 650,040	$ 1,250,000
Amounts offset - reverse repurchase agreement	0	(1,250,000)
Reverse repurchase agreements	650,040	0
Gross amounts -repurchase agreement	81,115,874	78,946,343
Amounts offset -repurchase agreement	0	(1,250,000)
Repurchase agreements outstanding	$ 81,115,874	$ 77,696,343