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SECURITIES - Additional Information (Details) - USD ($) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Residential Investment securities sold, carrying value	$ 2.8	$ 11.1	$ 50.2	$ 30.7

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- Definition

Carrying value of mortgage backed securities sold during period.

+ References

No definition available.

+ Details

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

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- References

No definition available.

+ Details

Name:	us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration