Repurchase Agreements (Details 2)	Sep. 30, 2012 Counterparty	Dec. 31, 2011 Counterparty
Repurchase Agreements
Number of counterparties	26	25
Threshold percent of stockholders' equity at risk with single counterparty to repurchase agreements or Linked Transactions	5.00%

Represents the percentage of stockholders' equity at risk with respect to repurchase agreements with any single counterparty, used as a threshold for disclosure.

No definition available.

Name:	mfa_RepurchaseAgreementCounterpartyRiskAsPercentageOfStockholdersEquity
Namespace Prefix:	mfa_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant