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Repurchase Agreements - Remaining Maturities (Parenthetical) (Detail)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Repurchase Agreements:    
Percentage of repurchase agreement 18.00%nly_DebtInstrumentRepurchaseRate 16.00%nly_DebtInstrumentRepurchaseRate