SECURED FINANCING - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Netted amounts	$ 55,475,420	$ 64,825,239	[1]
Repurchase agreements - weighted average remaining maturities	75 days	64 days
Repurchase agreement amount	$ 1,600,000
Remaining capacity of repurchase agreement	1,300,000
Fair value of collateral received for reverse repurchase agreements		$ 250,000
Secured financings and interest rate swaps - collateral held, estimated fair value	60,300,000	70,600,000
Secured financings and interest rate swaps - collateral held, accrued interest	$ 163,200	$ 196,900

[1] (1)Derived from the audited consolidated financial statements at December 31, 2020.