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v3.23.3

Expected Loss to be Paid (Recovered) - Additional Information (Details) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2023 USD ($) | Mar. 31, 2023 | Sep. 30, 2022 USD ($) | Jun. 30, 2023 USD ($) | Sep. 30, 2023 USD ($) Payment Curve scenario | Sep. 30, 2022 USD ($) | Dec. 31, 2022 USD ($) scenario | Jun. 30, 2022 USD ($) | Dec. 31, 2021 USD ($)
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Period of insured credit performance of guaranteed obligations (in some cases over) |  |  |  |  | 30 years |  |  |  |
Discount factor (as a percent) |  |  |  |  | 4.80% |  | 4.08% |  |
Net Expected Loss to be Paid (Recovered) | $ 490 |  | $ 727 | $ 560 | $ 490 | $ 727 | $ 522 | $ 442 | $ 411
Loss and LAE Reserve paid | 9 |  | 5 |  | 17 | 25 |  |  |
Expected LAE to be paid | 29 |  |  |  | 29 |  | 11 |  |
Net economic loss development (benefit) | 87 |  | (72) |  | $ 147 | (148) |  |  |
Additional loss recovery assumption, recovery period |  |  |  |  | 5 years |  |  |  |
First Lien |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Effect of recovery |  |  |  |  | $ 7 |  |  |  |
Second Lien |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Effect of recovery |  |  |  |  | 31 |  |  |  |
Public finance |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net Expected Loss to be Paid (Recovered) | 408 |  | 632 | 443 | 408 | 632 | 412 | 217 | 209
Net economic loss development (benefit) | 134 |  | 22 |  | 193 | (22) |  |  |
Other structured finance |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net Expected Loss to be Paid (Recovered) | 44 |  | 43 | 44 | 44 | 43 | $ 44 | 46 | 52
Net economic loss development (benefit) | 1 |  | 1 |  | $ 6 | 1 |  |  |
United States |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net expected loss to be paid after recoveries for representations and warranties, percent |  |  |  |  | 98.20% |  | 98.50% |  |
United States | Public finance |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net Expected Loss to be Paid (Recovered) | 399 |  | 626 | 433 | $ 399 | 626 | $ 403 | 210 | 197
Net economic loss development (benefit) | 135 |  | 24 |  | 193 | (16) |  |  |
United States | RMBS |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net Expected Loss to be Paid (Recovered) | 38 |  | 52 | $ 73 | 38 | 52 | $ 66 | $ 179 | $ 150
Net economic loss development (benefit) | (48) |  | (95) |  | (52) | (127) |  |  |
United States | RMBS | First Lien |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net economic loss development (benefit) | $ (12) |  | (38) |  | $ (12) | (34) |  |  |
Number of delinquent payments | Payment |  |  |  |  | 2 |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  |  |  |  | 36 months |  |  |  |
Projected loss assumptions, final CPR, period for voluntary prepayments to continue |  |  |  |  | 12 months |  |  |  |
Intermediate conditional default rate (as a percent) | 5.00% |  |  |  | 5.00% |  |  |  |
Number of scenarios weighted in estimating expected losses | scenario |  |  |  |  | 5 |  | 5 |  |
United States | RMBS | First Lien | Base Scenario |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered | 30.00% |  |  | 20.00% |  |  |  |  |
United States | RMBS | First Lien | More Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  | 10.00% |  |  |  |  |  |  |
United States | RMBS | First Lien | Least Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  | 50.00% |  |  |  |  |  |  |
United States | RMBS | First Lien | Base Scenario |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  |  |  |  | 36 months |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  | 12 months |  |  |  |
Final conditional default rate as a percentage of plateau conditional default rate |  |  |  |  | 5.00% |  |  |  |
Projected loss assumptions, final CPR, period for voluntary prepayments to continue |  |  |  |  | 1 year |  |  |  |
Default from delinquentor rate, term |  |  |  |  | 36 months |  |  |  |
Performing or projected to reperform, projection period |  |  |  |  | 36 months |  |  |  |
Projected loss assumptions, loss severity, subsequent period |  |  |  |  | 18 months |  |  |  |
Estimated loss severity rate, one through six months (as a percent) |  |  |  |  | 18 months |  |  |  |
Loss severity (as a percent) | 40.00% |  |  |  | 40.00% |  |  |  |
Projected loss assumptions, period to reach final loss severity rate |  |  |  |  | 2 years 6 months |  |  |  |
Final CPR | 15.00% |  |  |  | 15.00% |  |  |  |
United States | RMBS | First Lien | More Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  | 16 months |  |  |  |
Projected loss assumptions, period to reach final loss severity rate |  |  |  |  | 9 years |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  | 10.00% |  |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  |  |  |  | $ 28 |  |  |  |
United States | RMBS | First Lien | Least Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  |  |  |  | 30 months |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  | 8 months |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  | 50.00% |  |  |  |
Projected loss assumptions, increase (decrease) in expected loss to be paid, net |  |  |  |  | $ (20) |  |  |  |
Decrease in the plateau period used to calculate potential change in loss estimate (in months) |  |  |  |  | 6 months |  |  |  |
United States | RMBS | Second Lien |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Net economic loss development (benefit) | $ (36) |  | $ (57) |  | $ (40) | $ (93) |  |  |
Projected loss assumptions, period of consistent conditional default rate |  |  |  |  | 36 months |  |  |  |
Period of conditional default rate downward trend |  |  |  |  | 1 year |  |  |  |
Percent of original period of consistent conditional default rate |  |  |  |  | 5.00% |  |  |  |
Loss recovery assumption (as a percent) | 2.00% |  |  |  | 2.00% |  | 2.00% |  |
Liquidation rate | 40.00% |  |  | 30.00% | 40.00% |  |  |  |
Number of conditional default rate curves modeled in estimating losses | Curve |  |  |  |  | 5 |  |  |  |
United States | RMBS | Second Lien | More Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  | 10.00% |  |  |  |
United States | RMBS | Second Lien | Least Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Percent of deferred loan balances to be recovered |  |  |  |  | 80.00% |  |  |  |
United States | RMBS | Second Lien | Home Equity Line of Credit and Closed-end Mortgage |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Final CPR | 15.00% |  |  |  | 15.00% |  |  |  |
United States | RMBS | Second Lien | More Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Projected loss assumptions, CDR, plateau rate, projection period |  |  |  |  | 42 months |  |  |  |
Period from plateau to intermediate conditional default rate (in months) |  |  |  |  | 16 months |  |  |  |
Stress period (in months) |  |  |  |  | 58 months |  |  |  |
Increase in conditional default rate ramp down period |  |  |  |  | 4 months |  |  |  |
United States | RMBS | Second Lien | More Stressful Environment | Home Equity Line of Credit |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Change in estimate for increased conditional default rate plateau period |  |  |  |  | $ 89 |  |  |  |
United States | RMBS | Second Lien | Least Stressful Environment |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Stress period (in months) |  |  |  |  | 38 months |  |  |  |
Period of constant conditional default rate (in months) |  |  |  |  | 30 months |  |  |  |
Decreased conditional default rate ramp down period |  |  |  |  | 8 months |  |  |  |
Change in estimate for decreased prepayment rate, Percent |  |  |  |  | 10.00% |  |  |  |
United States | RMBS | Second Lien | Least Stressful Environment | Home Equity Line of Credit |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Change in estimate for decreased conditional default rate ramp down period |  |  |  |  | $ 120 |  |  |  |
United States | Home Equity Line of Credit |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Initial period for which borrower can pay only interest payments |  |  |  |  | 10 years |  |  |  |
United States | Home Equity Line of Credit Insured |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Initial period for which borrower can pay only interest payments |  |  |  |  | 15 years |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Risk free discount rate |  |  |  |  | 4.51% |  | 3.82% |  |
Maximum |  |  |  |  |  |  |  |  |
Schedule of Expected Losses to be Paid [Line Items] |  |  |  |  |  |  |  |  |
Risk free discount rate |  |  |  |  | 5.45% |  | 4.69% |  |

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- Definition Represents the percentage of assumed lifetime losses on a pool of insured obligations that is anticipated to be recovered. + References No definition available. + Details
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- Definition Represents the change in expected loss on a pool of insured obligations that would result from a decrease in the conditional default rate ramp period. + References No definition available. + Details
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- Definition [Line Items] for Schedule of Expected Losses to be Paid [Table] + References No definition available. + Details
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- Definition Represents the total stress period. + References No definition available. + Details
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- Details
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- Details
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- Details
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Period Type: |

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- Details
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- Details
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- Details
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Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementScenarioAxis=ago_BaseScenarioMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Balance Type: |
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- Details
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Balance Type: |
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- Details
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Balance Type: |
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- Details
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- Details
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- Details
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- Details
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