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This item represents the fair value of debt securities with an expected life of greater than ten years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeAfterTenYearsFairValue
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This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, with an expected life of greater than five years and less than ten years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeGreaterThanFiveAndLessThanTenYearsAmortizedCost
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
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This item represents the fair value of debt securities with an expected life of greater than five years and less than ten years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeGreaterThanFiveAndLessThanTenYearsFairValue
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This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, with an expected life of greater than one year and less than five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeGreaterThanOneAndLessThanFiveYearsAmortizedCost
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This item represents the fair value of debt securities with an expected life of greater than one year and less than five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeGreaterThanOneAndLessThanFiveYearsFairValue
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This item represents debt securities, at cost, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, if any, with an expected life of less than one year of the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeOfLessThanOneYearAmortizedCost
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Data Type:	xbrli:monetaryItemType
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This item represents the fair value of debt securities with an expected life of less than one year of the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

No definition available.

Name:	nly_AvailableForSaleSecuritiesDebtExpectedLifeOfLessThanOneYearFairValue
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Data Type:	xbrli:monetaryItemType
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Mortgage-Backed Securities Estimated Weighted Average Life Amortized Cost [Abstract]

No definition available.

Name:	nly_MortgageBackedSecuritiesEstimatedWeightedAverageLifeAmortizedCostAbstract
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Mortgage-Backed Securities Estimated Weighted Average Life Fair Value [Abstract]

No definition available.

Name:	nly_MortgageBackedSecuritiesEstimatedWeightedAverageLifeFairValueAbstract
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Data Type:	xbrli:stringItemType
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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481961/940-320-45-2