A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	mfa_AccumulatedOtherComprehensiveIncomeLossRollForward
Namespace Prefix:	mfa_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the weighted average interest rate paid related to derivatives.

+ References

No definition available.

+ Details

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

- Definition

Represents the weighted average interest rate received related to derivatives.

+ References

No definition available.

+ Details

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

- Definition

Other Comprehensive Income (Loss), Amortization of De-Designated Hedging Instruments, after Reclassification and Tax

+ References

No definition available.

+ Details

Name:	mfa_OtherComprehensiveIncomeLossAmortizationofDeDesignatedHedgingInstrumentsafterReclassificationandTax
Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, after tax, of accumulated gain (loss) on derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e637-108580

+ Details

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

- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Average variable interest rate related to the group of interest rate derivatives.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageVariableInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of realized and unrealized loss of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109998890-113959

+ Details

Name:	us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

- Definition

Amount of decrease in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5618551-113959

+ Details

Name:	us-gaap_DerivativeLossOnDerivative
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The net amount of nonoperating interest income (expense).

+ References

No definition available.

+ Details

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

- Definition

The carrying amount, as of the date of the latest financial statement presented, of all assets that are pledged as collateral for the payment of the related debt obligation primarily arising from secured borrowings, repurchase agreements or other current or potential obligations, where the secured party does not have the right by contract or custom to sell or re-pledge the collateral and therefore are not reclassified and separately reported in the statement of financial position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver3MonthsTo6MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver6MonthsTo12MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver12MonthsTo24MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver24MonthsTo36MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver36MonthsTo48MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Other Assets (Derivative Instruments) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
AOCI from derivative hedging instruments:
Balance at beginning of period	$ (7,176,000)	$ (28,114,000)	$ (22,675,000)	$ 3,121,000
Net loss on Swaps	0	(233,000)	(50,127,000)	(30,384,000)
Reclassification adjustment for losses/(gains) related to hedging instruments included in net income	7,176,000	(685,000)	72,802,000	(1,769,000)
Balance at end of period	0	(29,032,000)	0	(29,032,000)
Interest Rate Contract
Derivative [Line Items]
Assets pledged	0		0		$ 19,018,000
Swaps, at fair value
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 3,172,000,000
Weighted average fixed-pay rate	0.00%		0.00%		2.24%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.81%
Interest (expense)/income attributable to Swaps	$ 0	$ (322,000)	$ (3,359,000)	$ 1,561,000
Weighted average Swap rate paid	0.00%	2.29%	2.06%	2.32%
Weighted average Swap rate received	0.00%	2.24%	1.63%	2.40%
Swaps, at fair value \| Over 3 months to 6 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 200,000,000
Weighted average fixed-pay rate	0.00%		0.00%		2.05%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.70%
Swaps, at fair value \| Over 6 months to 12 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 1,430,000,000
Weighted average fixed-pay rate	0.00%		0.00%		2.30%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.77%
Swaps, at fair value \| Over 12 months to 24 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 1,300,000,000
Weighted average fixed-pay rate	0.00%		0.00%		2.11%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.86%
Swaps, at fair value \| Over 24 months to 36 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 20,000,000
Weighted average fixed-pay rate	0.00%		0.00%		1.38%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.90%
Swaps, at fair value \| Over 36 months to 48 months
Derivative [Line Items]
Aggregate notional amount of derivatives	$ 0		$ 0		$ 222,000,000
Weighted average fixed-pay rate	0.00%		0.00%		2.88%
Weighted Average Variable Interest Rate	0.00%		0.00%		1.84%
Swap \| Hedging
Derivative [Line Items]
Loss on derivative			$ 0
Swap \| Non-Hedging
Derivative [Line Items]
Loss on derivative		$ 929,000	4,300,000	$ 17,300,000
Realized loss		$ 3,700,000	9,400,000	$ 17,700,000
Agency MBS, at fair value \| Interest Rate Contract
Derivative [Line Items]
Assets pledged	$ 0		0		$ 2,241,000
Restricted cash \| Interest Rate Contract
Derivative [Line Items]
Assets pledged	$ 0		$ 0		$ 16,777,000