Contracts Accounted for as Insurance - Gross Premium Receivable Net of Commissions on Assumed Business Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Gross Premium Receivable Net of Ceding Commissions [Roll Forward]
Beginning balance	$ 1,286	$ 904
Less: Specialty insurance premium receivable	2	1
Financial guaranty insurance premiums receivable	1,284	903
Gross written premiums on new business, net of commissions	67	41
Gross premiums received, net of commissions	(60)	(54)
Adjustments:
Changes in the expected term	(4)	(4)
Accretion of discount, net of commissions on assumed business	3	1
Foreign exchange translation and remeasurement	(58)	9
FG insurance premiums receivable	1,232	896
Less: Specialty insurance premium receivable	1	1
Ending balance	1,233	897
Variable Interest Entity, Primary Beneficiary [Member]
Adjustments:
Ending balance	$ 7	$ 8

Financial Guarantee Insurance Contracts, Premium Receivable, Excluding Other Non Financial Guarantee

No definition available.

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

- Definition

Financial Guarantee Insurance Contracts, Premium Receivable, Specialty Insurance

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Represents gains or losses due to foreign exchange rate changes that relate to installment premium receivables on financial guaranty insurance contracts denominated in currencies other than U.S. dollar.

+ References

No definition available.

+ Details

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- Definition

Amount of the premium receivable for financial guarantee insurance contracts reported in the statement of financial position. This element primarily would consist of those financial guarantee insurance contracts which premiums are to be collected over the contract period, but may, on occasion, include premiums which are to be collected in full at inception, but which straddle a financial reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(1),(5)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

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

- Definition

Amount of new business written on financial guarantee insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash inflow from premiums received for premium receivable on financial guarantee insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 17
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3367-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivableProceedsFromPremiumsReceived
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accretion of the discount for premiums receivable on financial guarantee insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(4)(iii)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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- Definition

Amount of increase (decrease) in the premium receivable on financial guarantee insurance contracts from changes in the period of the contract.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(4)(i)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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