SECURED FINANCING - Mortgage Loans Payable (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Variable interest rate	90.00%	88.00%
Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 510,386	$ 511,056
Debt instrument face amount	516,113	516,834
Joint Ventures \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	316,380	316,275
Debt instrument face amount	$ 318,664	$ 318,664
Joint Ventures \| Commercial Mortgage Loan \| Minimum
Debt Instrument [Line Items]
Interest rate	4.03%	4.03%
Joint Ventures \| Commercial Mortgage Loan \| Maximum
Debt Instrument [Line Items]
Interest rate	4.96%	4.96%
Joint Ventures \| Commercial Mortgage Loan \| LIBOR
Debt Instrument [Line Items]
Long-term debt, gross	$ 16,125	$ 16,125
Debt instrument face amount	$ 16,125	$ 16,125
Variable interest rate	2.75%	2.75%
Maryland \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 95,364	$ 95,827
Debt instrument face amount	$ 97,187	$ 97,667
Maryland \| Commercial Mortgage Loan \| Minimum
Debt Instrument [Line Items]
Interest rate	2.34%	2.75%
Maryland \| Commercial Mortgage Loan \| Maximum
Debt Instrument [Line Items]
Interest rate	4.53%	4.96%
Texas \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 32,060	$ 32,189
Debt instrument face amount	$ 33,597	$ 33,735
Interest rate	3.28%	3.28%
Utah \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 9,744	$ 9,703
Debt instrument face amount	$ 9,706	$ 9,706
Interest rate	3.69%	3.69%
Utah \| Commercial Mortgage Loan \| LIBOR
Debt Instrument [Line Items]
Long-term debt, gross	$ 7,156	$ 7,279
Debt instrument face amount	$ 7,175	$ 7,201
Variable interest rate	3.50%	3.50%
Minnesota \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 13,390	$ 13,438
Debt instrument face amount	$ 13,425	$ 13,473
Interest rate	3.69%	3.69%
Tennessee \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 12,303	$ 12,328
Debt instrument face amount	$ 12,350	$ 12,350
Interest rate	4.01%	4.01%
Wisconsin \| Commercial Mortgage Loan
Debt Instrument [Line Items]
Long-term debt, gross	$ 7,864	$ 7,892
Debt instrument face amount	$ 7,884	$ 7,913
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/presentationRef
-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=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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=6876686&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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ 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

+ 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:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_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:	us-gaap_StatementGeographicalAxis=stpr_VA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_TX
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_UT
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_MN
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_TN
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_WI
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: