Interest Rate Risk - Narrative (Detail) (USD $) In Billions, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012
Interest Rate Risk:
Derivative instruments, notional amount	$ 52.4	$ 46.9

Notional Amount Of Interest Rate Derivatives Settled

No definition available.

Name:	nly_NotionalAmountOfInterestRateDerivativesSettled
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration