CRE Debt and Preferred Equity Investments (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Commercial Mortgage
Transaction to Real Estate Investments [Line Items]
Net origination fees	$ 4.8	$ 3.0

Mortgage Loans on Real Estate, Origination Fees, Net

No definition available.

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