Cover - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 17, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	1-13991
Entity Registrant Name	MFA FINANCIAL, INC.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	13-3974868
Entity Address, Address Line One	350 Park Avenue, 20th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10022
City Area Code	212
Local Phone Number	207-6400
Title of 12(g) Security	None
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 1.1
Entity Common Stock, Shares Outstanding (in shares)		452,058,867
Documents Incorporated by Reference	Portions of the registrant’s proxy statement to be filed with the Securities and Exchange Commission in connection with the Annual Meeting of Stockholders scheduled to be held on or about June 2, 2021, are incorporated by reference into Part III of this Annual Report on Form 10-K.
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Central Index Key	0001055160
Common Stock
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	MFA
Security Exchange Name	NYSE
7.50% Series B Cumulative Redeemable Preferred Stock, par value $0.01 per share
Entity Information [Line Items]
Title of 12(b) Security	7.50% Series B Cumulative RedeemablePreferred Stock, par value $0.01 per share
Trading Symbol	MFA/PB
Security Exchange Name	NYSE
6.50% Series C Cumulative Redeemable Preferred Stock, par value $0.01 per share
Entity Information [Line Items]
Title of 12(b) Security	6.50% Series C Cumulative RedeemablePreferred Stock, par value $0.01 per share
Trading Symbol	MFA/PC
Security Exchange Name	NYSE