Equity - Capital Requirements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulatory Capital Requirements [Abstract]
Capital Requirements, Minimum Capital, Percentage of Aggregate On-Balance Sheet Assets Included In Core Capital	2.75%
Capital Requirements, Minimum Capital, Percentage of Aggregate Off-Balance Sheet Assets Included In Core Capital	0.75%
Capital Requirements, Critical Capital, Percentage of Core Capital of Total Minimum Capital	50.00%
Capital Required for Capital Adequacy	$ 434.2	$ 421.3
Core Capital	531.3agm_CoreCapital	766.3agm_CoreCapital
Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy	$ 97.1	$ 345.0

Capital Requirements, Critical Capital, Percentage of Core Capital of Total Minimum Capital

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- Definition

Capital Requirements, Minimum Capital, Percentage of Aggregate Off-Balance Sheet Assets Included In Core Capital

+ References

No definition available.

+ Details

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- Definition

Capital Requirements, Minimum Capital, Percentage of Aggregate On-Balance Sheet Assets Included In Core Capital

+ References

No definition available.

+ Details

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- Definition

Core Capital (Total Equity minus Accumulated Other Comprehensive Income)

+ References

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- Definition

Excess Capital, Core Capital In Excess of Capital Required for Capital Adequacy

+ References

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- Definition

The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

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