SECURED FINANCING - Mortgage Loans Payable (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Variable interest rate	94.00%	92.00%
Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	$ 426,256,000	$ 485,005,000
Mortgage principal	429,650,000	490,631,000
Joint Ventures \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	316,686,000	316,566,000
Mortgage principal	$ 318,302,000	$ 318,562,000
Joint Ventures \| Commercial Mortgage Loans \| Minimum
Debt Instrument [Line Items]
Interest rate	4.03%	4.03%
Joint Ventures \| Commercial Mortgage Loans \| Maximum
Debt Instrument [Line Items]
Interest rate	4.96%	4.96%
Joint Ventures \| Commercial Mortgage Loans \| LIBOR
Debt Instrument [Line Items]
Mortgage carrying value	$ 16,607,000	$ 16,029,000
Mortgage principal	$ 16,325,000	$ 16,325,000
Variable interest rate	2.15%	2.15%
Virginia \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value		$ 82,940,000
Mortgage principal		$ 84,702,000
Virginia \| Commercial Mortgage Loans \| Minimum
Debt Instrument [Line Items]
Interest rate		2.34%
Virginia \| Commercial Mortgage Loans \| Maximum
Debt Instrument [Line Items]
Interest rate		4.55%
Virginia \| Commercial Mortgage Loans \| LIBOR
Debt Instrument [Line Items]
Mortgage carrying value	$ 24,464,000
Mortgage principal	$ 25,000,000
Variable interest rate	2.85%
Texas \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	$ 31,127,000	$ 31,667,000
Mortgage principal	$ 32,582,000	$ 33,167,000
Interest rate	3.28%	3.28%
Utah \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	$ 6,969,000	$ 7,077,000
Mortgage principal	$ 6,986,000	$ 7,096,000
Interest rate	3.69%	3.69%
Utah \| Commercial Mortgage Loans \| LIBOR
Debt Instrument [Line Items]
Mortgage carrying value	$ 9,706,000	$ 9,706,000
Mortgage principal	$ 9,706,000	$ 9,706,000
Variable interest rate	2.75%	3.50%
Minnesota \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	$ 13,039,000	$ 13,243,000
Mortgage principal	$ 13,072,000	$ 13,276,000
Interest rate	3.69%	3.69%
Wisconsin \| Commercial Mortgage Loans
Debt Instrument [Line Items]
Mortgage carrying value	$ 7,658,000	$ 7,777,000
Mortgage principal	$ 7,677,000	$ 7,797,000
Interest rate	3.69%	3.69%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: 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(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtInstrumentAxis=nly_CommercialMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=nly_JointVentureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_VA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_TX
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_UT
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_MN
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_WI
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: