Summary of Significant Accounting Policies (Financing Agreements) (Details)	12 Months Ended
Dec. 31, 2020
Accounting Policies [Abstract]
Repurchase financing period, low end of range	1 month
Repurchase financing period, high end of range	6 months
Repurchase agreements financing collateralized, period	12 months

Represents the high end of the range of the typical period for repurchase financing agreements.

No definition available.

Name:	mfa_RepurchaseAgreementsFinancingPeriodHighEndOfRange
Namespace Prefix:	mfa_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration