COVER PAGE - shares	9 Months Ended
	Sep. 30, 2020	Oct. 30, 2020
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	1-13447
Entity Registrant Name	ANNALY CAPITAL MANAGEMENT INC
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	22-3479661
Entity Address, Address Line One	1211 Avenue of the Americas
Entity Address, City or Town	New York,
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10036
City Area Code	212
Local Phone Number	696-0100
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		1,398,228,330
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001043219
Current Fiscal Year End Date	--12-31
Common Stock, par value $0.01 per share
Document Information [Line Items]
Title of Each Class	Common Stock, par value $0.01 per share
Trading Symbol	NLY
Name of Each Exchange on Which Registered	NYSE
7.50% Series D Cumulative Redeemable Preferred Stock
Document Information [Line Items]
Title of Each Class	7.50% Series D Cumulative Redeemable Preferred Stock
Trading Symbol	NLY.D
Name of Each Exchange on Which Registered	NYSE
6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Document Information [Line Items]
Title of Each Class	6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Trading Symbol	NLY.F
Name of Each Exchange on Which Registered	NYSE
6.50% Series G Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Document Information [Line Items]
Title of Each Class	6.50% Series G Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Trading Symbol	NLY.G
Name of Each Exchange on Which Registered	NYSE
6.75% Series I Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Document Information [Line Items]
Title of Each Class	6.75% Series I Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock
Trading Symbol	NLY.I
Name of Each Exchange on Which Registered	NYSE