Repurchase Agreements (Repricing Information) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	$ 9,139,821	$ 7,879,087
Repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	9,140,944	7,879,114
Less debt issuance costs	1,123	27
Total repurchase agreements less debt issuance costs	$ 9,139,821	$ 7,879,087
Weighted Average Interest Rate	2.99%	3.42%
Within 30 days \| Repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	$ 4,472,120	$ 6,747,166
Weighted Average Interest Rate	2.55%	3.35%
Over 30 days to 3 months \| Repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	$ 2,746,384	$ 368,857
Weighted Average Interest Rate	3.43%	3.10%
Over 3 months to 12 months \| Repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	$ 1,014,441	$ 763,091
Weighted Average Interest Rate	3.36%	4.18%
Over 12 months \| Repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Repurchase agreements	$ 907,999	$ 0
Weighted Average Interest Rate	3.44%	0.00%

Securities Sold Under Agreements To Repurchase, Net of Debt Issuance Costs

No definition available.

Name:	mfa_SecuritiesSoldUnderAgreementsToRepurchaseNetofDebtIssuanceCosts
Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num: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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=49176635&loc=d3e9760-107771

Reference 4: 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=6876686&loc=d3e534808-122878

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(a)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

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

- Details

Name:	mfa_AssetsSoldUnderAgreementsToRepurchaseRemainingPeriodBeforeInterestRateResetAxis=mfa_InterestRateResetWithin30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_AssetsSoldUnderAgreementsToRepurchaseRemainingPeriodBeforeInterestRateResetAxis=mfa_InterestRateResetOver30DaysTo3MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_AssetsSoldUnderAgreementsToRepurchaseRemainingPeriodBeforeInterestRateResetAxis=mfa_InterestRateResetOver3Monthsto12MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_AssetsSoldUnderAgreementsToRepurchaseRemainingPeriodBeforeInterestRateResetAxis=mfa_InterestRateResetOver12MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: