Organization (Details)	9 Months Ended
Sep. 30, 2012
Organization
Minimum annual REIT taxable income distributable to stockholders (as a percent)	90.00%

Represents the minimum percentage of annual taxable income which must be distributed to stockholders in order to comply with requirements of the federal tax law for treatment as a real estate investment trust.

No definition available.

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