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SECURITIES - Summary of Residential Securities and CMBS (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($)
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance January 1, 2021	$ 75,652,396
Purchases	11,834,619
Sales and transfers	(6,344,212)
Principal paydowns	(10,228,923)
(Amortization) / accretion	(307,699)
Fair value adjustment	(1,573,846)
Ending balance June 30, 2021	69,032,335
Residential Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance January 1, 2021	75,571,654
Purchases	11,684,613
Sales and transfers	(6,265,442)
Principal paydowns	(10,228,923)
(Amortization) / accretion	(307,987)
Fair value adjustment	(1,575,745)
Ending balance June 30, 2021	68,878,170
Commercial Securities
Debt Securities, Available-For-Sale [Roll Forward]
Beginning balance January 1, 2021	80,742
Purchases	150,006
Sales and transfers	(78,770)
Principal paydowns	0
(Amortization) / accretion	288
Fair value adjustment	1,899
Ending balance June 30, 2021	$ 154,165

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-SubTopic 210
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