Financing Agreements (Borrowings Under Financing Agreement And Associated Assets Pledged as Collateral) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of securities pledged as collateral under financing agreements	$ 5,414,137	$ 5,522,396
Non-mark-to-market financing secured by residential whole loans \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	786,951	1,156,125
Fair value of securities pledged as collateral under financing agreements	$ 1,343,544	$ 1,930,283
Weighted average haircut (percent)	40.85%	39.46%
Non-mark-to-market financing secured by real estate owned \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	$ 8,390	$ 3,088
Fair value of securities pledged as collateral under financing agreements	$ 21,161	$ 7,441
Weighted average haircut (percent)	60.04%	59.73%
Mark-to-market financing agreements secured by residential whole loans
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of securities pledged as collateral under financing agreements	$ 3,408,057	$ 4,140,961
Mark-to-market financing agreements secured by residential whole loans \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	847,922	1,113,553
Fair value of securities pledged as collateral under financing agreements	$ 1,362,771	$ 1,798,813
Weighted average haircut (percent)	35.14%	34.17%
Mark-to-market financing agreements secured by residential whole loans \| Carrying Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	$ 324,834
Fair value of securities pledged as collateral under financing agreements	$ 374,194
Weighted average haircut (percent)	13.16%
Mark-to-market financing agreements secured by securities at fair value \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	$ 0	$ 213,915
Fair value of securities pledged as collateral under financing agreements	$ 0	$ 399,999
Weighted average haircut (percent)	0.00%	41.16%
Mark-to-market financing agreements secured by securities at fair value \| Carrying Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	$ 178,357
Fair value of securities pledged as collateral under financing agreements	$ 302,835
Weighted average haircut (percent)	39.27%
Mark-to-market financing agreements secured by real estate owned
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of securities pledged as collateral under financing agreements	$ 90,998	$ 49,477
Mark-to-market financing agreements secured by real estate owned \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Carrying Value	10,144	10,609
Fair value of securities pledged as collateral under financing agreements	$ 40,617	$ 22,525
Weighted average haircut (percent)	69.71%	55.56%
Loan Securitization \| Fair Value
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of securities pledged as collateral under financing agreements		$ 141,900

The weighted average percentage amount by which the collateral value is contractually required to exceed the loan amount on the Company's repurchase agreements borrowings.

No definition available.

+ Details

Name:	mfa_HaircutOnSecuritiesWeightedAveragePercentage
Namespace Prefix:	mfa_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

Name:	us-gaap_PledgedAssetsSeparatelyReportedSecuritiesPledgedForRepurchaseAgreementsAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(1)(i))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(1)(ii)(A)(i))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=mfa_NonMarktoMarketFinancingFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=mfa_NonMarkToMarketFinancingSecuredByRealEstateOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=mfa_MarkToMarketFinancingAgreementsSecuredBySecuritiesAtFairValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=mfa_RealEstateOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=mfa_LoanSecuritizationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: