DERIVATIVE INSTRUMENTS - Summary of Characteristics of Interest Rate Swaps (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Interest rate swaps, at fair value
Derivative [Line Items]
Current Underlying Notional	$ 39,816,500,000	$ 34,329,650,000
Weighted Average Pay Rate	0.80%	0.92%
Weighted Average Receive Rate	0.34%	0.37%
Weighted Average Underlying Years to Maturity	3 years 3 months 10 days	3 years 11 months 8 days
Interest rate swaps, at fair value | LIBOR
Derivative [Line Items]
Notional Amount, Percentage	0.08	0.17
Interest rate swaps, at fair value | Federal funds index swap
Derivative [Line Items]
Notional Amount, Percentage	0.62	0.72
Interest rate swaps, at fair value | Secured Overnight Financing Rate (SOFR)
Derivative [Line Items]
Notional Amount, Percentage	0.30	0.11
Interest rate swaps, at fair value | 0 - 3 years
Derivative [Line Items]
Minimum Maturity Period	0 years	0 years
Maximum Maturity Period	3 years	3 years
Current Underlying Notional	$ 31,167,000,000	$ 23,680,150,000
Weighted Average Pay Rate	0.24%	0.27%
Weighted Average Receive Rate	0.06%	0.11%
Weighted Average Underlying Years to Maturity	1 year 9 months 18 days	1 year 11 months 15 days
Interest rate swaps, at fair value | 3 - 6 years
Derivative [Line Items]
Minimum Maturity Period	3 years	3 years
Maximum Maturity Period	6 years	6 years
Current Underlying Notional	$ 3,100,000,000	$ 3,600,000,000
Weighted Average Pay Rate	0.13%	0.18%
Weighted Average Receive Rate	0.06%	0.09%
Weighted Average Underlying Years to Maturity	4 years 1 month 17 days	4 years 2 months 15 days
Interest rate swaps, at fair value | 6 - 10 years
Derivative [Line Items]
Minimum Maturity Period	6 years	6 years
Maximum Maturity Period	10 years	10 years
Current Underlying Notional	$ 4,065,500,000	$ 5,565,500,000
Weighted Average Pay Rate	1.27%	1.40%
Weighted Average Receive Rate	0.65%	0.62%
Weighted Average Underlying Years to Maturity	7 years 9 months 7 days	7 years 9 months 3 days
Interest rate swaps, at fair value | Greater than 10 years
Derivative [Line Items]
Minimum Maturity Period	10 years	10 years
Current Underlying Notional	$ 1,484,000,000	$ 1,484,000,000
Weighted Average Pay Rate	3.06%	3.06%
Weighted Average Receive Rate	0.33%	0.36%
Weighted Average Underlying Years to Maturity	20 years 3 months 7 days	20 years 6 months 7 days
Forward starting pay fixed swaps
Derivative [Line Items]
Current Underlying Notional	$ 0	$ 0