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Summary of Characteristics of Interest Rate Swaps (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Derivative Instruments:    
Weighted average fixed rate 2.49%nly_InterestRateSwapsWeightedAveragePayRate 2.14%nly_InterestRateSwapsWeightedAveragePayRate
Forward Starting Pay Fixed Swaps    
Derivative Instruments:    
Notional amount 500,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember		  
Weighted average fixed rate 3.25%nly_InterestRateSwapsWeightedAveragePayRate
/ us-gaap_DerivativeInstrumentRiskAxis
= nly_ForwardStartingPayFixedSwapsMember