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Summary of Significant Accounting Policies - Equity-Based Compensation, Taxes, Derivatives (Details) (USD $)
12 Months Ended 36 Months Ended
Dec. 31, 2014
Component
Dec. 31, 2010
Dec. 31, 2013
Share based compensation      
Forfeiture rate assumption for equity based awards granted in 2009 and prior years (as a percent) 0.00%mfa_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeitureRate2009AndPrior    
Income Tax Disclosure      
Percentage of annual REIT taxable income distributed to stockholders 100.00%mfa_PercentageOfAnnualREITTaxableIncomeDistributedToStockholders    
Income tax expense $ 0us-gaap_IncomeTaxExpenseBenefit    
Derivative Instruments      
Number of components of linked transactions 2mfa_NumberOfComponentsOfLinkedTransactions    
Restricted Stock Units      
Share based compensation      
Vesting period of restricted share units (RSUs) 3 years    
Restricted Stock Units | Minimum      
Share based compensation      
Vesting period of restricted share units (RSUs)   2 years 1 year
Period for measuring market condition of award   2 years  
Restricted Stock Units | Maximum      
Share based compensation      
Vesting period of restricted share units (RSUs)   4 years 3 years
Period for measuring market condition of award   4 years