Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details 6)	9 Months Ended
Sep. 30, 2013 Component
Accounting Policies [Abstract]
Number of components in a linked transaction	2

Represents the number of components that make up a linked transaction which is not reported separately but evaluated on a combined basis and reported as a forward (derivative) contract.

No definition available.

Name:	mfa_NumberOfComponentsOfLinkedTransactions
Namespace Prefix:	mfa_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration