SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Unrealized Loss Position For:
Estimated Fair Value	$ 65,622,352	$ 75,652,396
Gross Unrealized Losses	(457,058)	(166,947)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated Fair Value	19,705,994	777,586
Gross Unrealized Losses	$ (292,272)	$ (2,030)
Number of Securities \| security	411	30
Agency Mortgage-Backed Securities \| Less than 12 months
Unrealized Loss Position For:
Estimated Fair Value	$ 19,701,354	$ 777,586
Gross Unrealized Losses	$ (292,136)	$ (2,030)
Number of Securities \| security	409	30
Agency Mortgage-Backed Securities \| 12 Months or more
Unrealized Loss Position For:
Estimated Fair Value	$ 4,640	$ 0
Gross Unrealized Losses	$ (136)	$ 0
Number of Securities \| security	2	0

Mortgage-Backed Securities in a continuous unrealized loss position [Line Items]

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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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-Section 45
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

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