Residential Whole Loans (Fair Value Components of Net Income) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Receivables [Abstract]
Net unrealized gains/(losses)	$ 6,226	$ 2,010	$ 38,313	$ (72,546)
Other income	(205)	2,900	(803)	2,147
Total	$ 6,021	$ 4,910	$ 37,510	$ (70,399)

Residential Whole Loans, Fair Value, Net Income (Loss), Coupon Payments, Realized Gains, and Other Income Received

No definition available.

Name:	mfa_ResidentialWholeLoansFairValueNetIncomeLossCouponPaymentsRealizedGainsAndOtherIncomeReceived
Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration