SECURITIES - Additional Information (Details) - USD ($) $ in Billions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Residential investment securities sold, carrying value	$ 25.5	$ 51.8

Carrying value of mortgage backed securities sold during period.

No definition available.

Name:	nly_MortgageBackedSecuritiesSoldCarryingValue
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration