(Derivative Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Rate Contract
Derivative [Line Items]
Assets Pledged	$ 19,018	$ 32,803
Swaps, at fair value
Derivative [Line Items]
Notional Amount	$ 3,172,000	$ 3,217,000
Average maturity term of swaps	16 months
Weighted average fixed-pay rate	2.24%	2.42%
Weighted Average Variable Interest Rate	1.81%	2.56%
Interest expense attributable to Swaps	$ 927	$ 3,780	$ 24,524
Weighted average Swap rate paid	2.28%	2.12%	1.98%
Weighted average Swap rate received	2.24%	1.96%	1.07%
Swaps, at fair value \| Within 30 days
Derivative [Line Items]
Notional Amount	$ 0	$ 0
Weighted average fixed-pay rate	0.00%	0.00%
Weighted Average Variable Interest Rate	0.00%	0.00%
Swaps, at fair value \| Over 30 days to 3 months
Derivative [Line Items]
Notional Amount	$ 0	$ 100,000
Weighted average fixed-pay rate	0.00%	1.71%
Weighted Average Variable Interest Rate	0.00%	2.50%
Swaps, at fair value \| Over 3 months to 6 months
Derivative [Line Items]
Notional Amount	$ 200,000	$ 100,000
Weighted average fixed-pay rate	2.05%	1.71%
Weighted Average Variable Interest Rate	1.70%	2.50%
Swaps, at fair value \| Over 6 months to 12 months
Derivative [Line Items]
Notional Amount	$ 1,430,000	$ 0
Weighted average fixed-pay rate	2.30%	0.00%
Weighted Average Variable Interest Rate	1.77%	0.00%
Swaps, at fair value \| Over 12 months to 24 months
Derivative [Line Items]
Notional Amount	$ 1,300,000	$ 1,630,000
Weighted average fixed-pay rate	2.11%	2.27%
Weighted Average Variable Interest Rate	1.86%	2.50%
Swaps, at fair value \| Over 24 months to 36 months
Derivative [Line Items]
Notional Amount	$ 20,000	$ 800,000
Weighted average fixed-pay rate	1.38%	2.57%
Weighted Average Variable Interest Rate	1.90%	2.64%
Swaps, at fair value \| Over 36 months to 48 months
Derivative [Line Items]
Notional Amount	$ 222,000	$ 0
Weighted average fixed-pay rate	2.88%	0.00%
Weighted Average Variable Interest Rate	1.84%	0.00%
Swaps, at fair value \| Over 48 months to 60 months
Derivative [Line Items]
Notional Amount	$ 0	$ 417,000
Weighted average fixed-pay rate	0.00%	2.88%
Weighted Average Variable Interest Rate	0.00%	2.63%
Swaps, at fair value \| Over 84 months
Derivative [Line Items]
Notional Amount	$ 0	$ 170,000
Weighted average fixed-pay rate	0.00%	3.00%
Weighted Average Variable Interest Rate	0.00%	2.66%
Swaps, at fair value \| Hedging
Derivative [Line Items]
Notional Amount	$ 3,200,000
Swaps, at fair value \| Minimum \| LIBOR
Derivative [Line Items]
Derivative, variable interest rate, term	1 month
Swaps, at fair value \| Maximum
Derivative [Line Items]
Maximum maturity term of swaps	47 months
Swaps, at fair value \| Maximum \| LIBOR
Derivative [Line Items]
Derivative, variable interest rate, term	3 months
Swaps \| Non-Hedging
Derivative [Line Items]
Loss on derivative	$ 16,500	$ 9,600
Realized loss	17,700
Agency MBS, at fair value \| Interest Rate Contract
Derivative [Line Items]
Assets Pledged	2,241	2,735
Restricted cash \| Interest Rate Contract
Derivative [Line Items]
Assets Pledged	$ 16,777	$ 30,068

Derivative, Variable Interest Rate, Term

No definition available.

Name:	mfa_DerivativeVariableInterestRateTerm
Namespace Prefix:	mfa_
Data Type:	xbrli:durationItemType
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

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 remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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=109980867&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=109980867&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=109980867&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=109980867&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=109980867&loc=SL5579245-113959

+ Details

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

- Definition

Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- 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_DerivativeMaturityWithin30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver30DaysTo3MonthsMember
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:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver48MonthsTo60MonthsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mfa_DerivativeByMaturityAxis=mfa_DerivativeMaturityOver84MonthsMember
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:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
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_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: