Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair value transfers between levels	$ 0	$ 0	$ 0	$ 0

The amount of transfers between levels of the fair value hierarchy.

No definition available.

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