Goodwill - Narrative (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	3 Months Ended
Sep. 30, 2012	Mar. 31, 2012 Merganser [Member]
Goodwill [Line Items]
Earn out payment	$ 13,387	$ 13,387

Payment made subsequent to business combination upon achieving of certain criteria outlined in the agreement.

No definition available.

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