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LOANS - Fair Value and Unpaid Principal of Residential Mortgage Loan Portfolio (Details) - Residential Mortgage - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Mortgage Loans on Real Estate [Line Items] |  |
Fair value | $ 2,737,388 | $ 2,454,637
Unpaid principal balance | $ 2,686,557 | $ 2,425,657

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- Definition Mortgage loans fair value disclosure. + References No definition available. + Details
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Namespace Prefix: | nly_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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- Definition This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719 + Details
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Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |