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Equity Compensation, Employment Agreements and Other Benefit Plans (Savings Plan) (Details) - Savings Plan - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employer contribution percentage on first 3 percent of eligible compensation deferred by employees		100.00%
Percentage of eligible compensation deferred by employees qualifying for 100 percent matching contribution		3.00%
Employer contribution percentage on next 2 percent of eligible compensation deferred by employees		50.00%
Percentage of eligible compensation deferred by employees qualifying for 50 percent matching contribution		2.00%
Percentage of employer matching contributions that vest immediately		100.00%
Expenses for matching contributions	$ 371,000	$ 480,000	$ 503,500

X

- Definition

Represents the percentage of the employer's matching contribution on the first tier of eligible compensation deferred by employees.

+ References

No definition available.

+ Details

Name:	mfa_DeferredCompensationArrangementWithIndividualEmployerMatchingContributionOnFirstTierOfEligibleCompensation
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of the employer's matching contribution on the second tier of eligible compensation deferred by employees.

+ References

No definition available.

+ Details

Name:	mfa_DeferredCompensationArrangementWithIndividualEmployerMatchingContributionOnSecondTierOfEligibleCompensation
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of the first tier of eligible compensation deferred by employees and receiving the highest level of employer match.

+ References

No definition available.

+ Details

Name:	mfa_DeferredCompensationArrangementWithIndividualPercentageOfFirstEligibleCompensationForEmployerContributionMatch
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the percentage of the second tier of eligible compensation deferred by employees and receiving the second level of employer match.

+ References

No definition available.

+ Details

Name:	mfa_DeferredCompensationArrangementWithIndividualPercentageOfNextEligibleCompensationForEmployerContributionMatch
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The percentage of employer matching contributions under a deferred compensation arrangement that vest immediately.

+ References

No definition available.

+ Details

Name:	mfa_MatchingContributionByEmployerVestingPercent
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of contributions made by employer into fund established for purposes of making future disbursement to individual in accordance with deferred compensation arrangement.

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualShareBasedPaymentsByTypeOfDeferredCompensationAxis=mfa_SavingsPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: