Variable Interest Entities - Schedule of Redeemable Noncontrolling Interest of Consolidated Investment Vehicle (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($)
Redeemable Noncontrolling Interest, Equity, Other, Fair Value, Rollforward [Roll Forward]
Beginning balance	$ 7
Reallocation of ownership interests	(2)
Contributions to investment vehicles	5
Net loss	(2)
March 31,	$ 8

Redeemable Noncontrolling Interest, Equity, Other, Contributions to Investment Vehicles

No definition available.

Name:	ago_RedeemableNoncontrollingInterestEquityOtherContributionstoInvestmentVehicles
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Redeemable Noncontrolling Interest, Equity, Other, Fair Value, Rollforward [Roll Forward]

+ References

No definition available.

+ Details

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

- Definition

Redeemable Noncontrolling Interest, Equity, Other, Reallocation of Ownership Interests

+ References

No definition available.

+ Details

Name:	ago_RedeemableNoncontrollingInterestEquityOtherReallocationofOwnershipInterests
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of Net Income (Loss) attributable to redeemable noncontrolling interest.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.19)
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

+ Details

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

- Definition

Fair value of noncontrolling interests which are redeemable by the parent entity and classified as other equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 3A
-Subparagraph (12)(c)
-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 3A
-Subparagraph (16)(c)
-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 3A
-Subparagraph 14
-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section S99
-Paragraph 3A
-Subparagraph 15
-URI http://asc.fasb.org/extlink&oid=65877616&loc=SL6540498-122764

+ Details

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