Residential Whole Loans (Narrative) (Details) - USD ($) $ in Billions	Dec. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Total residential whole loans	$ 7.4	$ 4.7

Loans, Including Fair Value Loans, Credit Impaired Loans and Other Loans at Carrying Value

No definition available.

Name:	mfa_LoansIncludingFairValueLoansCreditImpairedLoansandOtherLoansatCarryingValue
Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant