Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions |
9 Months Ended | |
|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | $ 1,536 | $ 1,068 |
| Personal Lines | Personal auto business | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | 1,300 | 910 |
| Personal Lines | Personal property | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | 75 | |
| Commercial Lines | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | 210 | 80 |
| Short-Duration Insurance Contract, Accident Year 2025 | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | 975 | |
| Short-Duration Insurance Contract, Accident Year 2024 | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | $ 325 | 660 |
| Short-Duration Insurance Contract, Accident Year 2023 | ||
| Liability for Claims and Claims Adjustment Expense [Line Items] | ||
| Prior years | $ 190 | |