LOANS - Corporate Debt Amortized Costs Basis by Risk Rating and Vintage Year (Details) - Corporate Loans $ in Thousands	Dec. 31, 2020 USD ($)
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	$ 2,239,930
2020	537,681
2019	448,615
2018	729,382
2017	421,723
2016	68,191
2015	34,338
Accrued interest receivable	11,000
Deferred loan fees on unfunded loans	1,400
Performing
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	1,760,669
2020	499,186
2019	400,873
2018	402,712
2017	355,369
2016	68,191
2015	34,338
Performing - Special Mention
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	337,386
2020	38,495
2019	0
2018	283,464
2017	15,427
2016	0
2015	0
Substandard
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	141,875
2020	0
2019	47,742
2018	43,206
2017	50,927
2016	0
2015	0
Doubtful
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	0
2020	0
2019	0
2018	0
2017	0
2016	0
2015	0
Loss
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Total	0
2020	0
2019	0
2018	0
2017	0
2016	0
2015	$ 0

Debt Securities, Held-to-maturity, before Allowance for Credit Loss

No definition available.

Name:	nly_DebtSecuritiesHeldtomaturitybeforeAllowanceforCreditLoss
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans and Leases Receivable, Deferred Loan Fees on Unfunded Loans

+ References

No definition available.

+ Details

Name:	nly_LoansAndLeasesReceivableDeferredLoanFeesOnUnfundedLoans
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

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

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated more than five years prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated four years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated in current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedInCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated in fiscal year prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated three years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated two years prior to current fiscal year.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_InterestReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nly_PerformingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_UnlikelyToBeCollectedFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: