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Long-Term Debt and Credit Facilities - Narrative (Details) |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 25, 2023 USD ($) | Aug. 21, 2023 USD ($) | Aug. 16, 2023 | Sep. 27, 2021 USD ($) | Aug. 20, 2021 USD ($) | May 26, 2021 USD ($) | Jun. 20, 2014 USD ($) | Dec. 20, 2006 USD ($) period | Nov. 22, 2006 USD ($) rISK extension | May 18, 2004 USD ($) | Dec. 31, 2023 USD ($) Trust | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Jul. 09, 2021 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  |  |  | $ 1,746,000,000 | $ 1,726,000,000 |  |
Net proceeds from issuance of debt |  |  |  |  |  |  |  |  |  |  | 345,000,000 | 0 | $ 889,000,000 |
Repayments of long-term debt |  |  |  |  |  |  |  |  |  |  | 330,000,000 | 2,000,000 | 620,000,000 |
Loss on extinguishment of debt |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 175,000,000 |
Interest expense |  |  |  |  |  |  |  |  |  |  | 90,000,000 | 81,000,000 | 87,000,000 |
AGUS (Issuer | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, fair value disclosure |  |  |  |  |  |  |  |  |  |  |  | 154,000,000 | 154,000,000 |
AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on extinguishment of debt |  |  |  |  |  |  |  |  |  |  |  |  | 175,000,000 |
Loss on extinguishment of debt, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 138,000,000 |
Acceleration of unamortized fair value adjustments |  |  |  |  |  |  |  |  |  |  |  |  | 156,000,000 |
AGM and AGC | AGM CPS securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum stock purchase obligation of each custodial trust |  |  |  |  |  |  |  |  |  |  | 200,000,000 |  |  |
Aggregate maximum stock purchase obligation of the custodial trusts |  |  |  |  |  |  |  |  |  |  | $ 50,000,000 |  |  |
AGC | AGM CPS securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of custodial trusts | Trust |  |  |  |  |  |  |  |  |  |  | 4 |  |  |
Assured Guaranty Municipal Corp | AGM CPS securities |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of custodial trusts | Trust |  |  |  |  |  |  |  |  |  |  | 4 |  |  |
Junior Subordinated Debt | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  | $ 300,000,000 |  | $ 146,000,000 | 146,000,000 |  |
Interest rate, added to base rate (as a percent) |  |  |  |  |  |  |  |  | 2.33% |  |  |  |  |
Effective interest rate of debt (as a percent) |  |  |  |  |  |  |  |  | 6.40% |  |  |  |  |
Number of times interest payment may be deferred |  |  |  |  |  |  |  |  | 1 |  |  |  |  |
Number of consecutive periods for which interest payments may be deferred | rISK |  |  |  |  |  |  |  |  | 1 |  |  |  |  |
Period for which interest payment may be deferred (in years) |  |  |  |  |  |  |  |  | 10 years |  |  |  |  |
Number of times repayment date may be extended | extension |  |  |  |  |  |  |  |  | 4 |  |  |  |  |
Repayment date extension measurement period |  |  |  |  |  |  |  |  | 5 years |  |  |  |  |
Notes Payable, Other Payables | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 430,000,000
5% Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment for debt extinguishment |  |  |  |  |  |  |  |  |  |  |  |  | $ 19,000,000 |
5% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  | 5.00% |  |  |  | 5.00% |  |  |
Principal |  |  |  |  |  |  | $ 500,000,000 |  |  |  | $ 0 | 330,000,000 |  |
Net proceeds from issuance of debt |  |  |  |  |  |  | $ 495,000,000 |  |  |  |  |  |  |
Repayments of long-term debt | $ 330,000,000 |  |  | $ 170,000,000 |  |  |  |  |  |  |  |  |  |
3.6% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  |  |  |  |  | 3.60% |  |  |
Principal |  |  |  |  | $ 400,000,000 |  |  |  |  |  | $ 400,000,000 | 400,000,000 |  |
Net proceeds from issuance of debt |  |  |  |  | $ 395,000,000 |  |  |  |  |  |  |  |  |
Redemption price percentage |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  | 0.30% |  |  |  |  |  |  |  |  |  |  |
3.6% Senior Notes | Senior Notes | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  | 3.60% |  |  |  |  |  |  |  |  |
Senior Notes 6.125 Percent | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  | 6.125% |  |  |  |  |  |  |  |  | 6.125% |  |  |
Principal |  | $ 350,000,000 |  |  |  |  |  |  |  |  | $ 350,000,000 | 0 |  |
Net proceeds from issuance of debt |  | $ 345,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Redemption price percentage |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  | 0.30% |  |  |  |  |  |  |  |  |  |  |
3.15% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  |  |  |  |  | 3.15% |  |  |
Principal |  |  |  |  |  | $ 500,000,000 |  |  |  |  | $ 500,000,000 | 500,000,000 |  |
Net proceeds from issuance of debt |  |  |  |  |  | $ 494,000,000 |  |  |  |  |  |  |  |
Redemption price percentage |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  | 0.25% |  |  |  |  |  |  |  |  |  |  |
3.15% Senior Notes | Senior Notes | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  | 3.15% |  |  |  |  |  |  |  |
7% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  |  |  |  | 700.00% | 700.00% |  |  |
Principal |  |  |  |  |  |  |  |  |  | $ 200,000,000 | $ 200,000,000 | $ 200,000,000 |  |
Net proceeds from issuance of debt |  |  |  |  |  |  |  |  |  | $ 197,000,000 |  |  |  |
Effective interest rate of debt (as a percent) |  |  |  |  |  |  |  |  |  |  | 6.40% |  |  |
Enhanced Junior Subordinated Debentures | Junior Subordinated Debt | AGC |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | $ 150,000,000 |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  |  |  |  |  |  | 2.64% |  |  |  |  |  |
Number of times interest payment may be deferred |  |  |  |  |  |  |  | 1 |  |  |  |  |  |
Number of consecutive periods for which interest payments may be deferred | period |  |  |  |  |  |  |  | 1 |  |  |  |  |  |
Period for which interest payment may be deferred (in years) |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |
Enhanced Junior Subordinated Debentures | Junior Subordinated Debt | AGMH |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period prior to final repayment date before which debt cannot be repaid, redeemed, repurchased or defeased (in years) |  |  |  |  |  |  |  |  | 20 years |  |  |  |  |

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