The interest rate for investments that have an interest rate. For fixed rate investments, this indicates the fixed interest rate. If the investment has a variable interest rate, the rate stated here may be the rate that is currently relevant. In this case the "Investment, Interest Rate is Current Rate for Variable Rate Investment Flag", "Investment, Interest Rate is Discount Rate at Purchase Flag", "Investment, Interest Rate Reflects Current Yield Flag" should be used. If one of these flags are not appropriate to explain the interest rate that is indicated, then a new flag should be added or an explanation should be included in "Investment, Additional Information".

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 12

-Sentence Column A

+ Details

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Namespace Prefix:	invest_
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- Details

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- Definition

Represents the interest expense on mortgage backed securities in linked transactions (MBS purchase and repurchase financing) accounted for as derivative forward contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

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- Definition

Represents the amortized cost of mortgage backed securities in linked transactions (MBS purchase and repurchase financing) accounted for as derivative forward contracts.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Represents the fair value of mortgage backed securities in linked transactions (MBS purchase and repurchase financing) accounted for as derivative forward contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfa_
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- Definition

Represents the principal balance of mortgage backed securities in linked transactions (MBS purchase and repurchase financing) accounted for as derivative forward contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Represents the carrying value as of the balance sheet date of repurchase financing agreements related to MBS purchases, which are accounted for as forward purchase agreements and are netted against the carrying value of the associated MBS.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfa_
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- Definition

Represents unrealized net (losses)/gains and net interest income from linked transactions of the entity.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfa_
Data Type:	xbrli:monetaryItemType
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- Definition

Interest rate on the amount to be repaid to the counterparty to the repurchase agreement when the carrying amount (or market value, if higher than the carrying amount) of securities or other assets sold under the repurchase agreement exceeds 10 percent of total assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph m

-Subparagraph 1

-Article 4

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

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- Definition

Represents interest income, including any amortization and accretion (as applicable) of discounts and premiums, derived from securities backed by a pool of mortgages or trust deeds. Mortgage-backed securities might represent mortgage pass-through securities, collateralized mortgage obligations (CMO), real estate mortgage investment conduits (REMICs), CMO and REMIC residuals, and stripped mortgage-backed securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 65

-Paragraph 34

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.2)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Mortgage-Backed Securities

-URI http://asc.fasb.org/extlink&oid=6517959

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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Derivatives (Details 2) (Linked Transactions, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Linked Repurchase Agreements
Balance	$ 36,423		$ 36,423		$ 170,897
Weighted Average Interest Rate (as a percent)	1.61%		1.61%		1.88%
Linked MBS
Fair Value	49,058		49,058		225,969
Amortized Cost	45,658		45,658		237,595
Par/Current Face	54,690		54,690		289,536
Weighted Average Coupon Rate (as a percent)			4.32%		4.74%
Components of the gain on Linked Transactions
Interest income attributable to MBS underlying Linked Transactions	812	4,631	4,386	21,475
Interest expense attributable to linked repurchase agreement borrowings underlying Linked Transactions	(168)	(864)	(965)	(3,938)
Change in fair value of Linked Transactions included in earnings	2,533	(3,034)	8,023	(7,567)
Unrealized net gains and net interest income from Linked Transactions	3,177	733	11,444	9,970
Rated AAA
Linked MBS
Fair Value					29,057
Amortized Cost					29,917
Par/Current Face					30,675
Weighted Average Coupon Rate (as a percent)					3.31%
Rated AA
Linked MBS
Fair Value	14,544		14,544		17,427
Amortized Cost	13,728		13,728		16,858
Par/Current Face	14,120		14,120		17,297
Weighted Average Coupon Rate (as a percent)			5.00%		5.00%
Rated A
Linked MBS
Fair Value	3,137		3,137
Amortized Cost	2,641		2,641
Par/Current Face	3,486		3,486
Weighted Average Coupon Rate (as a percent)			0.79%
Rated BBB
Linked MBS
Fair Value	8,210		8,210		41,825
Amortized Cost	7,340		7,340		42,419
Par/Current Face	9,022		9,022		49,781
Weighted Average Coupon Rate (as a percent)			2.61%		2.81%
Rated BB
Linked MBS
Fair Value	7,257		7,257
Amortized Cost	6,855		6,855
Par/Current Face	7,639		7,639
Weighted Average Coupon Rate (as a percent)			2.74%
Rated CCC
Linked MBS
Fair Value					20,782
Amortized Cost					20,988
Par/Current Face					26,680
Weighted Average Coupon Rate (as a percent)					4.42%
Rated CC
Linked MBS
Fair Value					43,644
Amortized Cost					47,060
Par/Current Face					61,470
Weighted Average Coupon Rate (as a percent)					6.00%
Rated C
Linked MBS
Fair Value					32,870
Amortized Cost					36,934
Par/Current Face					45,857
Weighted Average Coupon Rate (as a percent)					5.20%
D-rated
Linked MBS
Fair Value	15,910		15,910
Amortized Cost	15,094		15,094
Par/Current Face	20,423		20,423
Weighted Average Coupon Rate (as a percent)			5.81%
Not Rated
Linked MBS
Fair Value					40,364
Amortized Cost					43,419
Par/Current Face					57,776
Weighted Average Coupon Rate (as a percent)					5.54%
Within 30 days
Linked Repurchase Agreements
Balance	36,423		36,423		141,719
Weighted Average Interest Rate (as a percent)	1.61%		1.61%		1.89%
Over 30 days to 90 days
Linked Repurchase Agreements
Balance					$ 29,178
Weighted Average Interest Rate (as a percent)					1.81%