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Summary of Characteristics of Interest Rate Swaps (Parenthetical) (Detail) (Forward Starting Pay Fixed Swaps, USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Forward Starting Pay Fixed Swaps
    
Derivative Instruments:    
Notional amount $ 3,000,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember		 $ 500,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember
Weighted average fixed rate 1.88%nly_InterestRateSwapsWeightedAveragePayRate
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember		 3.25%nly_InterestRateSwapsWeightedAveragePayRate
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember