| Cover Page | Dec. 31, 2020 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 31, 2020 |
| Entity Registrant Name | Annaly Capital Management Inc |
| Entity Incorporation, State or Country Code | MD |
| Entity File Number | 1-13447 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001043219 |
| Amendment Flag | false |
| Common Stock, par value $0.01 per share | |
| Document Information [Line Items] | |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | NLY |
| Security Exchange Name | NYSE |
| 6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 6.95% Series F Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock |
| Trading Symbol | NLY.F |
| Security Exchange Name | NYSE |
| 6.50% Series G Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 6.50% Series G Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock |
| Trading Symbol | NLY.G |
| Security Exchange Name | NYSE |
| 6.75% Series I Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock | |
| Document Information [Line Items] | |
| Title of 12(b) Security | 6.75% Series I Fixed-to-Floating Rate Cumulative Redeemable Preferred Stock |
| Trading Symbol | NLY.I |
| Security Exchange Name | NYSE |
