Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Dec. 31, 2018 USD ($) risk	Dec. 31, 2017 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding	$ 6,182	$ 6,207
Net Par Outstanding	$ 241,802	264,952
Number of Risks \| risk	9,413
BIG [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 10,003	11,900
Net par outstanding	157	338
Net Par Outstanding	$ 10,160	$ 12,238
Number of Risks, Financial Guaranty Insurance \| risk	312	335
Number of Risks, Credit Derivative \| risk	15	19
Number of Risks \| risk	327	354
BIG [Member] \| BIG 1 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 2,981	$ 4,301
Net par outstanding	77	227
Net Par Outstanding	$ 3,058	$ 4,528
Number of Risks, Financial Guaranty Insurance \| risk	128	139
Number of Risks, Credit Derivative \| risk	6	7
Number of Risks \| risk	134	146
BIG [Member] \| BIG 2 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 932	$ 1,344
Net par outstanding	5	17
Net Par Outstanding	$ 937	$ 1,361
Number of Risks, Financial Guaranty Insurance \| risk	39	46
Number of Risks, Credit Derivative \| risk	1	3
Number of Risks \| risk	40	49
BIG [Member] \| BIG 3 [Member]
Schedule of Insured Financial Obligations [Line Items]
Net Par Outstanding, Financial Guaranty Insurance	$ 6,090	$ 6,255
Net par outstanding	75	94
Net Par Outstanding	$ 6,165	$ 6,349
Number of Risks, Financial Guaranty Insurance \| risk	145	150
Number of Risks, Credit Derivative \| risk	8	9
Number of Risks \| risk	153	159

Represents the number of unique credits in the insured portfolio accounted for as credit derivative.

No definition available.

Name:	ago_FinancialGuarantyContractsAccountedForAsCreditDerivativesNumberOfContracts
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations that are accounted for as credit derivatives, net of protection purchased.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of unique credits in the insured portfolio.

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, after netting out ceded contracts.

+ References

No definition available.

+ Details

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

- Definition

This element represents the amount of insured contractual payments outstanding attributable to the principal of the underlying insured obligations for financial guaranty insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guaranty insurance contracts, before adjusting for the effect of consolidating VIEs.

+ References

No definition available.

+ Details

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

- Definition

[Line Items] for Schedule of Insured Financial Obligations [Table]

+ References

No definition available.

+ Details

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

- Definition

This element represents the number of issued and outstanding contracts for each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 9
-Subparagraph (a)(5)(i)
-URI http://asc.fasb.org/extlink&oid=99382878&loc=SL5749324-161292

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 55
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=99379334&loc=SL5751133-161288

+ Details

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Balance Type:	na
Period Type:	instant

- Details

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