Lease Commitments And Contingencies - Narrative (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Lease Commitments and Contingencies:
Aggregate future net minimum lease payments	$ 6,356

The total of future contractually required net payments on leases defined as operating.

No definition available.

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