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SECURITIES - Summary of Residential Securities and CMBS (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	$ 114,833,580
Purchases	19,317,457
Sales and transfers	(47,585,510)
Principal paydowns	(9,307,384)
(Amortization) / accretion	(886,958)
Fair value adjustment	1,434,558
Ending balance	77,805,743
Residential Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	114,560,557
Purchases	19,317,457
Sales and transfers	(47,391,453)
Principal paydowns	(9,302,451)
(Amortization) / accretion	(887,336)
Fair value adjustment	1,447,767
Ending balance	77,744,541
Commercial Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance	273,023
Purchases	0
Sales and transfers	(194,057)
Principal paydowns	(4,933)
(Amortization) / accretion	378
Fair value adjustment	(13,209)
Ending balance	$ 61,202

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