SECURITIES - Summary of Residential Securities and CMBS (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	$ 114,833,580
Purchases	33,107,404
Sales and transfers	(52,571,156)
Principal paydowns	(19,536,638)
(Amortization) / accretion	(1,373,838)
Fair value adjustment	1,193,044
Ending balance	75,652,396
Residential Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	114,560,557
Purchases	33,082,119
Sales and transfers	(52,367,095)
Principal paydowns	(19,531,705)
(Amortization) / accretion	(1,374,490)
Fair value adjustment	1,202,268
Ending balance	75,571,654
Commercial Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	273,023
Purchases	25,285
Sales and transfers	(204,061)
Principal paydowns	(4,933)
(Amortization) / accretion	652
Fair value adjustment	(9,224)
Ending balance	$ 80,742

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Debt Securities, Available-For-Sale [Roll Forward]

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash inflow from sale, maturity, prepayment and call of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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