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Long-Term Debt - Schedule of long-term debt (Details) - USD ($) | Dec. 31, 2020 | Nov. 15, 2020 | Sep. 30, 2020 | May 29, 2020 | Apr. 02, 2020 | Dec. 31, 2019 | Mar. 31, 2019
--- | --- | --- | --- | --- | --- | --- | ---
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 1,993,000,000 |  |  |  |  | $ 1,493,000,000 |
Discount and Issue Costs | (30,000,000) |  |  |  |  | (20,000,000) |
Carrying Value | 1,963,000,000 |  |  |  |  | 1,473,000,000 |
AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Carrying Value | 1,963,000,000 |  |  |  |  | 1,473,000,000 |
Senior Notes | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | 1,318,000,000 |  |  |  |  | 1,018,000,000 |
Discount and Issue Costs | (11,000,000) |  |  |  |  | (5,000,000) |
Carrying Value | 1,307,000,000 |  |  |  |  | 1,013,000,000 |
Senior Notes | 4.50% Senior Notes due June 2047 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | 590,000,000 |  |  |  |  | 590,000,000 |
Discount and Issue Costs | (2,000,000) |  |  |  |  | (2,000,000) |
Carrying Value | $ 588,000,000 |  |  |  |  | 588,000,000 |
Interest rate on debt instruments | 4.50% |  |  |  |  |  |
Senior Notes | 3.50% Senior Notes due August 2026 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 425,000,000 |  |  |  |  | 425,000,000 |
Discount and Issue Costs | (3,000,000) |  |  |  |  | (3,000,000) |
Carrying Value | $ 422,000,000 |  |  |  |  | 422,000,000 |
Interest rate on debt instruments | 3.50% |  |  |  |  |  |
Senior Notes | 5.25% Senior Notes due April 2030 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 300,000,000 |  |  |  | $ 300,000,000 | 0 |
Discount and Issue Costs | (6,000,000) |  |  |  |  | 0 |
Carrying Value | $ 294,000,000 |  |  |  |  | 0 |
Interest rate on debt instruments | 5.25% |  |  |  | 5.25% |  |
Senior Notes | Other | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 3,000,000 |  |  |  |  | 3,000,000 |
Discount and Issue Costs | 0 |  |  |  |  | 0 |
Carrying Value | 3,000,000 |  |  |  |  | 3,000,000 |
Subordinated Debentures | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | 675,000,000 |  |  |  |  | 475,000,000 |
Discount and Issue Costs | (19,000,000) |  |  |  |  | (15,000,000) |
Carrying Value | 656,000,000 |  |  |  |  | 460,000,000 |
Subordinated Debentures | 4.50% Subordinated Debentures due September 2060 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | 200,000,000 |  | $ 200,000,000 |  |  | 0 |
Discount and Issue Costs | (5,000,000) |  |  |  |  | 0 |
Carrying Value | $ 195,000,000 |  |  |  |  | 0 |
Interest rate on debt instruments | 4.50% |  | 4.50% |  |  |  |
Subordinated Debentures | 5.125% Subordinated Debentures due December 2059 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 200,000,000 |  |  |  |  | 200,000,000 |
Discount and Issue Costs | (6,000,000) |  |  |  |  | (6,000,000) |
Carrying Value | $ 194,000,000 |  |  |  |  | $ 194,000,000 |
Interest rate on debt instruments | 5.125% |  |  |  |  | 5.125% |
Subordinated Debentures | 5.625% Subordinated Debentures due June 2060 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 150,000,000 |  |  | $ 150,000,000 |  | $ 0 |
Discount and Issue Costs | (4,000,000) |  |  |  |  | 0 |
Carrying Value | $ 146,000,000 |  |  |  |  | 0 |
Interest rate on debt instruments | 5.625% |  |  | 5.625% |  |  |
Subordinated Debentures | 5.875% Subordinated Debentures due March 2059 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 125,000,000 |  |  |  |  | 125,000,000 |
Discount and Issue Costs | (4,000,000) |  |  |  |  | (4,000,000) |
Carrying Value | $ 121,000,000 |  |  |  |  | 121,000,000 |
Interest rate on debt instruments | 5.875% |  |  |  |  |  | 5.875%
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG |  |  |  |  |  |  |
Summary of Carrying value of long-term debt |  |  |  |  |  |  |
Principal | $ 0 | $ 150,000,000 |  |  |  | 150,000,000 |
Discount and Issue Costs | 0 |  |  |  |  | (5,000,000) |
Carrying Value | $ 0 |  |  |  |  | $ 145,000,000 |
Interest rate on debt instruments | 6.00% | 6.00% |  |  |  |  |

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