Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedAccretiononNetEarnedPremium
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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For financial guarantee insurance contracts, the undiscounted amount of premium revenue expected to be recognized over the term of the contract period.

No definition available.

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedNetEarnedPremiumtobeRecognized
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

For financial guarantee insurance contracts, the amount of premium revenue expected to be recognized beyond the twentieth year following the statement of financial position reporting date.

No definition available.

Name:	ago_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenuetobeRecognizedBeyondYear20
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

For financial guarantee insurance contracts, the aggregate amount of premium related to the premium receivable which is expected to be collected beyond the twentieth year following the statement of financial position reporting date.

No definition available.

Name:	ago_FinancialGuaranteeInsuranceContractsPremiumReceivablePremiumExpectedtobeCollectedBeyondYear20
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of unearned premium revenue in the statement of financial position for financial guaranty insurance contracts, net of the unexpired portion of premiums ceded and before adjustment for loss claims paid but not yet expensed.

No definition available.

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

Future Net Premiums to be Earned

No definition available.

Name:	ago_FutureNetPremiumsToBeEarnedAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Future Premiums to be Collected

No definition available.

Name:	ago_FuturePremiumsToBeCollectedAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of premium revenue under financial guarantee insurance contract to be recognized in fourth fiscal quarter following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479032/944-605-50-4

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in eleventh through fifteenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 4
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479032/944-605-50-4

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThan10AndWithin15YearsFromBalanceSheetDate
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Amount of premium revenue under financial guarantee insurance contract to be recognized in sixteenth through twentieth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThan15AndWithin20YearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanOneAndWithinTwoYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanThreeAndWithinFourYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of premium revenue under financial guarantee insurance contract to be recognized in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognizedMoreThanTwoAndWithinThreeYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in fourth fiscal quarter following current year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedDuringFourthQuarterFollowingBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in eleventh through fifteenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThan10AndWithin15YearsFromBalanceSheetDate
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Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in sixteenth through twentieth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThan15AndWithin20YearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in sixth through tenth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThanFiveAndWithin10YearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThanFourAndWithinFiveYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThanOneAndWithinTwoYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Amount of premium receivable under financial guarantee insurance contract to be collected in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThanThreeAndWithinFourYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of premium receivable under financial guarantee insurance contract to be collected in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumExpectedToBeCollectedMoreThanTwoAndWithinThreeYearsFromBalanceSheetDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of premium receivables under financial guarantee insurance contracts.

No definition available.