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Contracts Accounted for as Credit Derivatives - Net Change in Fair Value of Credit Derivatives Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2022 | Mar. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |
Realized gains (losses) and other settlements | $ (1) | $ 1
Net unrealized gains (losses) | (2) | (20)
Fair value gains (losses) on credit derivatives | $ (3) | $ (19)

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- Definition Represents the premiums received and receivable, ceding commissions, and the losses paid and payable. This element refers to the gain (loss) included in earnings. + References No definition available. + Details
Name: | ago_CreditDerivativeRealizedGainLossandOtherSettlements
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959 + Details
Name: | us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 + Details
Name: | us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration