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CRE Debt and Preferred Equity Investments (Parenthetical) (Detail) (Commercial Mortgage, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Commercial Mortgage
    
Transaction to Real Estate Investments [Line Items]    
Net origination fees $ 4.0nly_MortgageLoansOnRealEstateOriginationFeesNet
/ us-gaap_InvestmentTypeAxis
= nly_CommercialMortgageMember		 $ 3.0nly_MortgageLoansOnRealEstateOriginationFeesNet
/ us-gaap_InvestmentTypeAxis
= nly_CommercialMortgageMember