Summary of Significant Accounting Policies (Repurchase Agreements) (Details)	9 Months Ended
Sep. 30, 2020
Accounting Policies [Abstract]
Repurchase financing period, low end of range	1 month
Repurchase financing period, high end of range	6 months
Repurchase financings collateralized by residential whole loans, high end	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