CRE Debt and Preferred Equity Investments (Parenthetical) (Detail) (Commercial Mortgage, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Commercial Mortgage
Transaction to Real Estate Investments [Line Items]
Net origination fees	$ 4.9	$ 4.9

Mortgage Loans on Real Estate, Origination Fees, Net

No definition available.

Name:	nly_MortgageLoansOnRealEstateOriginationFeesNet
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant