Investments (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Investments
Percent of Total (as a percent)	100.00%	100.00%
Amortized Cost, total investment portfolio	$ 10,701.7	$ 10,330.0
Gross Unrealized Gains, total investment portfolio	352.1	260.2
Gross Unrealized Losses, total investment portfolio	(84.0)	(132.3)
Estimated Fair Value, total investment portfolio	10,969.8	10,457.9
AOCI Gain (Loss) on Securities with OTTI	0.9	(11.4)
Percentage of government agency obligations (as a percent)	63.00%	64.00%
Unrealized gains (losses) included in AOCI for recognized OTTI, net of tax	2.9	5.6
Unrealized gains (losses) included in AOCI for not recognized OTTI, net of tax	199.3	115.3
Fixed maturity securities
Investments
Percent of Total (as a percent)	90.00%	90.00%
Amortized Cost, total fixed maturity securities	9,596.1	9,274.7
Gross Unrealized Gains, total fixed maturity securities	352.1	259.9
Gross Unrealized Losses, total fixed maturity securities	(84.0)	(132.3)
Estimated Fair Value, total fixed maturity securities	9,864.2	9,402.3
AOCI Gain (Loss) on Securities with OTTI	0.9	(11.4)
U.S. government and agencies
Investments
Percent of Total (as a percent)	9.00%	10.00%
Amortized Cost, total fixed maturity securities	926.1	1,000.3
Gross Unrealized Gains, total fixed maturity securities	49.4	48.3
Gross Unrealized Losses, total fixed maturity securities	(0.2)	(0.4)
Estimated Fair Value, total fixed maturity securities	975.3	1,048.2
Obligations of state and political subdivisions
Investments
Percent of Total (as a percent)	47.00%	48.00%
Amortized Cost, total fixed maturity securities	5,038.7	4,922.0
Gross Unrealized Gains, total fixed maturity securities	162.3	99.9
Gross Unrealized Losses, total fixed maturity securities	(16.8)	(62.0)
Estimated Fair Value, total fixed maturity securities	5,184.2	4,959.9
AOCI Gain (Loss) on Securities with OTTI	1.8	(1.4)
Corporate securities
Investments
Percent of Total (as a percent)	10.00%	9.00%
Amortized Cost, total fixed maturity securities	1,045.7	980.1
Gross Unrealized Gains, total fixed maturity securities	28.7	25.2
Gross Unrealized Losses, total fixed maturity securities	(8.9)	(12.8)
Estimated Fair Value, total fixed maturity securities	1,065.5	992.5
AOCI Gain (Loss) on Securities with OTTI	(0.2)	0.2
RMBS
Investments
Percent of Total (as a percent)	11.00%	11.00%
Amortized Cost, total fixed maturity securities	1,208.8	1,158.9
Gross Unrealized Gains, total fixed maturity securities	59.9	56.5
Gross Unrealized Losses, total fixed maturity securities	(51.2)	(44.3)
Estimated Fair Value, total fixed maturity securities	1,217.5	1,171.1
AOCI Gain (Loss) on Securities with OTTI	(23.0)	(8.6)
CMBS
Investments
Percent of Total (as a percent)	5.00%	4.00%
Amortized Cost, total fixed maturity securities	498.8	365.7
Gross Unrealized Gains, total fixed maturity securities	17.7	14.8
Gross Unrealized Losses, total fixed maturity securities	(0.4)	(1.4)
Estimated Fair Value, total fixed maturity securities	516.1	379.1
AOCI Gain (Loss) on Securities with OTTI	2.7	2.5
Asset-backed securities
Investments
Percent of Total (as a percent)	5.00%	5.00%
Amortized Cost, total fixed maturity securities	540.4	498.2
Gross Unrealized Gains, total fixed maturity securities	26.0	9.9
Gross Unrealized Losses, total fixed maturity securities	(5.5)	(5.2)
Estimated Fair Value, total fixed maturity securities	560.9	502.9
AOCI Gain (Loss) on Securities with OTTI	19.6	(4.1)
Foreign government securities
Investments
Percent of Total (as a percent)	3.00%	3.00%
Amortized Cost, total fixed maturity securities	337.6	349.5
Gross Unrealized Gains, total fixed maturity securities	8.1	5.3
Gross Unrealized Losses, total fixed maturity securities	(1.0)	(6.2)
Estimated Fair Value, total fixed maturity securities	344.7	348.6
Short-term investments.
Investments
Percent of Total (as a percent)	10.00%	10.00%
Amortized Cost, total investment portfolio	1,105.6	1,055.3
Gross Unrealized Gains, total investment portfolio		0.3
Estimated Fair Value, total investment portfolio	$ 1,105.6	$ 1,055.6

Represents the amount of other-than-temporary impairments for fixed maturity securities available-for-sale, pre-tax, that are reflected in accumulated other comprehensive income (loss).

Name:	ago_AccumulatedOtherComprehensiveIncomeLossGainLossOnInvestmentsIncludingOtherThanTemporaryImpairments
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents unrealized gains (losses), net of tax, included in AOCI on securities for which the entity had not recognized OTTI.

+ Details

Name:	ago_AccumulatedOtherComprehensiveIncomeLossNetOfTaxUnrealizedGainLossOfNoOtherThanTemporaryImpairmentLossesInvestments
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of other-than-temporary impairments for fixed maturity securities available-for-sale, after-tax, that are reflected in accumulated other comprehensive income (loss).

+ Details

Name:	ago_AccumulatedOtherComprehensiveIncomeLossNetOfTaxUnrealizedGainsLossesOfOtherThanTemporaryImpairmentLossesInvestments
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents government agency obligations as a percentage of the aggregate amortized cost of mortgage backed securities.

+ Details

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

- Definition

Represents individual investment categories as a percentage of the aggregate amortized cost of investments.

+ Details

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

- Definition

Represents the cost of investments, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ Details

Name:	ago_InvestmentsAmortizedCost
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cumulative gain resulting from the difference between the fair value and the carrying value of investments held at each balance sheet date.

+ Details

Name:	ago_UnrealizedGainOnInvestments
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cumulative loss resulting from the difference between the fair value and the carrying value of investments held at each balance sheet date.

+ Details

Name:	ago_UnrealizedLossOnInvestments
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent reporting 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:	invest_InvestmentLineItems
Namespace Prefix:	invest
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-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 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13, 19

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-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 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6956420&loc=d3e24584-111560

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant