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, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Gross amounts - reverse repurchase agreements	$ 100,000	$ 650,040
Amounts offset - reverse repurchase agreements	(100,000)	0
Reverse repurchase agreements	0	650,040
Gross amounts - repurchase agreements	101,840,728	81,115,874
Amounts offset - repurchase agreements	(100,000)	0
Repurchase agreements	$ 101,740,728	$ 81,115,874