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Long-Term Debt and Credit Facilities - Narrative (Details) $ in Millions |  |  |  |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 25, 2023 USD ($) | Aug. 21, 2023 USD ($) | Aug. 20, 2021 USD ($) | May 26, 2021 USD ($) | Dec. 20, 2006 USD ($) period | Nov. 22, 2006 USD ($) extension rISK | May 18, 2004 USD ($) | Dec. 31, 2024 USD ($) Trust | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  |  |  | $ 1,746 | $ 1,746 |
Issuance of long-term debt, net of issuance costs |  |  |  |  |  |  |  | 0 | 345 | $ 0
Repayments of long-term debt |  |  |  |  |  |  |  | 0 | 330 | 2
Interest expense |  |  |  |  |  |  |  | 91 | 90 | $ 81
AGUS (Issuer | AGMH |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument, fair value disclosure |  |  |  |  |  |  |  | $ 154 | 154 |
AG (U.S. domiciled) | AGM CPS securities |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Number of custodial trusts | Trust |  |  |  |  |  |  |  | 8 |  |
Maximum stock purchase obligation of each custodial trust |  |  |  |  |  |  |  | $ 400 |  |
Aggregate maximum stock purchase obligation of the custodial trusts |  |  |  |  |  |  |  | 50 |  |
Junior Subordinated Debt | AGMH |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  |  | $ 300 |  | $ 146 | 146 |
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 |  |  |  |
5% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Repayments of long-term debt | $ 330 |  |  |  |  |  |  |  |  |
AGUS 3.6% Senior Notes | Senior Notes | AGMH |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  | 3.60% |  |  |  |  |  |  |
AGUS 3.6% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  |  | 3.60% |  |
Principal |  |  | $ 400 |  |  |  |  | $ 400 | 400 |
Issuance of long-term debt, net of issuance costs |  |  | $ 395 |  |  |  |  |  |  |
Redemption price percentage |  |  | 100.00% |  |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  | 0.30% |  |  |  |  |  |  |
AGUS 6.125% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  | 6.125% |  |  |  |  |  | 6.125% |  |
Principal |  | $ 350 |  |  |  |  |  | $ 350 | 350 |
Issuance of long-term debt, net of issuance costs |  | $ 345 |  |  |  |  |  |  |  |
Redemption price percentage |  | 100.00% |  |  |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  | 0.30% |  |  |  |  |  |  |  |
AGUS 3.15% Senior Notes | Senior Notes | AGMH |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  | 3.15% |  |  |  |  |  |
AGUS 3.15% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  |  | 3.15% |  |
Principal |  |  |  | $ 500 |  |  |  | $ 500 | 500 |
Issuance of long-term debt, net of issuance costs |  |  |  | $ 494 |  |  |  |  |  |
Redemption price percentage |  |  |  | 100.00% |  |  |  |  |  |
Interest rate, added to base rate (as a percent) |  |  |  | 0.25% |  |  |  |  |  |
AGUS 7% Senior Notes | Senior Notes | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Interest rate of debt (as a percent) |  |  |  |  |  |  | 700.00% | 700.00% |  |
Principal |  |  |  |  |  |  | $ 200 | $ 200 | $ 200 |
Issuance of long-term debt, net of issuance costs |  |  |  |  |  |  | $ 197 |  |  |
Effective interest rate of debt (as a percent) |  |  |  |  |  |  |  | 6.40% |  |
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 |  |  |  |
Enhanced Junior Subordinated Debentures | Junior Subordinated Debt | AGC |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |
Principal |  |  |  |  | $ 150 |  |  |  |  |
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 |  |  |  |  |

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