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SECURITIES - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Residential Investment securities sold, carrying value	$ 3,000,000,000.0	$ 41,900,000,000
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Impairment, available-for-sale debt security	$ 400,000	$ 0

X

- Definition

Carrying value of mortgage backed securities sold during period.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=121553693&loc=SL6284422-111562

+ Details

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Balance Type:	debit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: