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Schedule IV - Mortgage Loans on Real Estate (Parenthetical) (Detail) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Mortgage Loans on Real Estate [Line Items]    
Face Amount $ 1,520,972,000us-gaap_MortgageLoansOnRealEstateFaceAmountOfMortgages  
Carrying Amount 1,518,165,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages 1,583,969,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
Term B Notes [Member] | First Mortgage [Member] | Retail Site [Member] | COLORADO    
Mortgage Loans on Real Estate [Line Items]    
Face Amount 3,800us-gaap_MortgageLoansOnRealEstateFaceAmountOfMortgages
/ invest_InvestmentAxis
= nly_TermBNotesMember
/ us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis
= us-gaap_FirstMortgageMember
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ nly_QuantitativeDisclosuresByEligibleItemOrGroupAxis
= stpr_CO		  
Carrying Amount $ 1,900us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ invest_InvestmentAxis
= nly_TermBNotesMember
/ us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis
= us-gaap_FirstMortgageMember
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ nly_QuantitativeDisclosuresByEligibleItemOrGroupAxis
= stpr_CO