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SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized Loss Position For:
Estimated fair value	$ 75,652,396	$ 114,833,580
Gross unrealized losses	(166,947)	(291,441)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	777,586	19,007,519
Gross unrealized losses	$ (2,030)	$ (129,385)
Number of securities | security	30	491
Less than 12 months | Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	$ 777,586	$ 7,388,239
Gross unrealized losses	$ (2,030)	$ (24,056)
Number of securities | security	30	139
12 Months or more | Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	$ 0	$ 11,619,280
Gross unrealized losses	$ 0	$ (105,329)
Number of securities | security	0	352

X

- Definition

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

+ References

No definition available.

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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=121645371&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
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

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-Section 50
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-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

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- Definition

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=121645371&loc=d3e27290-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922888-210455

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