SECURED FINANCING - Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Disclosure [Abstract]
Gross amounts - reverse repurchase agreements	$ 250,000	$ 100,000
Amounts offset - reverse repurchase agreements	(250,000)	(100,000)
Reverse repurchase agreements	0	0
Gross amounts - repurchase agreements	65,075,239	101,840,728
Amounts offset - repurchase agreements	(250,000)	(100,000)
Repurchase agreements	$ 64,825,239	$ 101,740,728