Represents the maximum percentage points added to the reference rate to compute the variable rate on the financial instrument, in the event the auction process does not attract sufficient clearing bids.

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Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the amounts of gains and losses from fair value changes in committed capital securities included in earnings during the period.

+ Details

Name:	ago_FairValueGainLossOnCommittedCapitalSecurities
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the number of custodial trusts with whom the entity entered into put agreements.

+ Details

Name:	ago_PutAgreementsNumberOfCustodialTrusts
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage points added to the reference rate to compute the variable rate for income distributions on the financial instrument.

+ Details

Name:	ago_SharesSubjectToMandatoryRedemptionBasisSpreadOnVariableRate
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The reference rate for the variable rate for income distributions on the financial instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ Details

Name:	ago_SharesSubjectToMandatoryRedemptionVariableRateBasis
Namespace Prefix:	ago
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of days in a rate setting interval for auction market preferred securities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 02

-Paragraph 29

+ Details

Name:	us-gaap_AuctionMarketPreferredSecuritiesStockSeriesRateSettingInterval
Namespace Prefix:	us-gaap
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

Name:	us-gaap_SharesSubjectToMandatoryRedemptionBySettlementTermsLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount that would be paid, determined under the conditions specified in the contract, if as of the reporting date, the holder of the share has the right to redeem the shares.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 150

-Paragraph 27

-Subparagraph a

-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 480

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6940734&loc=d3e22047-110879

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The maximum amount that the issuer could be required to pay to redeem the instrument by physical settlement, if applicable.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 150

-Paragraph 27

-Subparagraph c

-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 480

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6940734&loc=d3e22047-110879

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt and Credit Facilities (Details 3) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Apr. 30, 2008 AGC CCS securities	Sep. 30, 2011 AGC CCS securities	Sep. 30, 2010 AGC CCS securities	Sep. 30, 2011 AGC CCS securities	Sep. 30, 2010 AGC CCS securities	Apr. 08, 2008 AGC CCS securities	Apr. 08, 2005 AGC CCS securities	Sep. 30, 2011 AGM CPS securities	Sep. 30, 2010 AGM CPS securities	Sep. 30, 2011 AGM CPS securities D	Sep. 30, 2010 AGM CPS securities	Jun. 30, 2003 AGM CPS securities
Committed Capital Securities
Number of custodial trusts											4					4
Maximum stock purchase obligation of each custodial trust											$ 50,000,000					$ 50,000,000
Aggregate maximum stock purchase obligation of the custodial trusts											200,000,000					200,000,000
Rate basis for income distributions					one-month LIBOR			one-Month LIBOR						one-month LIBOR
Rate added to basis for income distributions (as a percent)						2.50%		2.50%		1.10%
Auction interval (in days)														28
Maximum rate added to basis for income distributions, based on auction process (as a percent)												2.00%		2.00%
Fair value gain (loss)	$ 2,443,000	$ (5,548,000)	$ 3,538,000	$ 5,770,000		$ 700,000	$ (2,200,000)	$ 1,300,000	$ 5,100,000			$ 1,700,000	$ (3,300,000)	$ 2,200,000	$ 700,000