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Investments - Fixed Maturity Securities and Short Term Investments (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Jun. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023
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Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 100.00% |  | 100.00% |  |  |
Amortized Cost | $ 6,492 |  | $ 6,827 |  |  |
Allowance for Credit Losses | (54) | $ (47) | (60) | $ (66) | $ (69) | $ (77)
Gross Unrealized Gains | 130 |  | 44 |  |  |
Gross Unrealized Losses | (290) |  | (442) |  |  |
Estimated Fair Value | $ 6,278 |  | $ 6,369 |  |  |
Government agency obligations as a percentage of total mortgage backed securities | 74.00% |  | 68.00% |  |  |
Obligations of state and political subdivisions |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 28.00% |  | 30.00% |  |  |
Amortized Cost | $ 1,808 |  | $ 2,032 |  |  |
Allowance for Credit Losses | (13) |  | (14) |  |  |
Gross Unrealized Gains | 36 |  | 25 |  |  |
Gross Unrealized Losses | (76) |  | (103) |  |  |
Estimated Fair Value | $ 1,755 |  | $ 1,940 |  |  |
U.S. government and agencies |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 1.00% |  | 1.00% |  |  |
Amortized Cost | $ 51 |  | $ 72 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 1 |  | 1 |  |  |
Gross Unrealized Losses | (4) |  | (6) |  |  |
Estimated Fair Value | $ 48 |  | $ 67 |  |  |
Corporate securities |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 46.00% |  | 38.00% |  |  |
Amortized Cost | $ 2,977 |  | $ 2,586 |  |  |
Allowance for Credit Losses | (7) |  | (7) |  |  |
Gross Unrealized Gains | 73 |  | 9 |  |  |
Gross Unrealized Losses | (120) |  | (206) |  |  |
Estimated Fair Value | $ 2,923 |  | $ 2,382 |  |  |
RMBS |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 10.00% |  | 9.00% |  |  |
Amortized Cost | $ 670 |  | $ 657 |  |  |
Allowance for Credit Losses | (24) |  | (21) |  |  |
Gross Unrealized Gains | 8 |  | 2 |  |  |
Gross Unrealized Losses | (51) |  | (71) |  |  |
Estimated Fair Value | $ 603 |  | $ 567 |  |  |
CMBS |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 3.00% |  | 3.00% |  |  |
Amortized Cost | $ 184 |  | $ 189 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 2 |  | 0 |  |  |
Gross Unrealized Losses | (1) |  | (3) |  |  |
Estimated Fair Value | $ 185 |  | $ 186 |  |  |
CLOs |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 8.00% |  | 9.00% |  |  |
Amortized Cost | $ 506 |  | $ 615 |  |  |
Allowance for Credit Losses | (10) |  | (1) |  |  |
Gross Unrealized Gains | 5 |  | 6 |  |  |
Gross Unrealized Losses | (29) |  | (9) |  |  |
Estimated Fair Value | $ 472 |  | $ 611 |  |  |
Others |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 3.00% |  | 9.00% |  |  |
Amortized Cost | $ 196 |  | $ 593 |  |  |
Allowance for Credit Losses | 0 |  | (17) |  |  |
Gross Unrealized Gains | 3 |  | 1 |  |  |
Gross Unrealized Losses | 0 |  | (30) |  |  |
Estimated Fair Value | 199 |  | 547 |  |  |
Others | Affiliated Entity |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Amortized Cost | 48 |  | 41 |  |  |
Estimated Fair Value | $ 49 |  | $ 42 |  |  |
Non-U.S. government securities |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Percent of Total | 1.00% |  | 1.00% |  |  |
Amortized Cost | $ 100 |  | $ 83 |  |  |
Allowance for Credit Losses | 0 |  | 0 |  |  |
Gross Unrealized Gains | 2 |  | 0 |  |  |
Gross Unrealized Losses | (9) |  | (14) |  |  |
Estimated Fair Value | $ 93 |  | $ 69 |  |  |

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