Repurchase Agreements (Details 3) (Credit Suisse, USD $) In Thousands, unless otherwise specified	Sep. 30, 2011 M
Credit Suisse
Repurchase agreements counterparty risk
Counterparty rating	A/Aa2/AA-
Amount at Risk	$ 502,921
Weighted Average Months to Maturity for Repurchase Agreements (in months)	2
Counterparty risk as percent of Stockholders' Equity, minimum (as a percent)	19.00%

Represents the rating by S&P, Moody's and Fitch, Inc. for any counterparty for repurchase agreements and/or Linked Transactions for which the Company had greater than 10% of stockholders' equity at risk in the aggregate.

Name:	mfa_CounterpartyRating
Namespace Prefix:	mfa
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the minimum percentage of stockholders' equity at risk with respect to repurchase agreements with any single counterparty.

+ Details

Name:	mfa_RepurchaseAgreementCounterpartyRiskMinimumAsPercentageOfStockholdersEquity
Namespace Prefix:	mfa
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount at risk under repurchase agreements is defined as the excess of carrying amount (or market value, if higher than the carrying amount or if there is no carrying amount) of the securities or other assets sold under agreement to repurchase, including accrued interest plus any cash or other assets on deposit to secure the repurchase obligation, over the amount of the repurchase liability (adjusted for accrued interest).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 4

-Section 08

-Paragraph m

-Subparagraph 1

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(B)(iii))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

+ Details

Name:	us-gaap_RepurchaseAgreementCounterpartyAmountAtRisk
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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.

+ Details

Name:	us-gaap_RepurchaseAgreementCounterpartyLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This equals the weighted average maturity (weighted by amount of the agreement) of all repurchase agreements with this counterparty.

+ References

+ Details

Name:	us-gaap_RepurchaseAgreementCounterpartyWeightedAverageMaturityOfAgreements
Namespace Prefix:	us-gaap
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant