Reserves for Losses and Loss Adjustment Expenses - Schedule of Impact from (Favorable) Unfavorable Development of Prior Accident Years' Loss and LAE Reserves on Each Reporting Segment (Details) - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2024 |
|
| Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] | ||
| Total unfavorable (favorable) prior-year development | $ 1.0 | $ 24.8 |
| Casualty Lines | ||
| Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] | ||
| Total unfavorable (favorable) prior-year development | 0.8 | 0.7 |
| Specialty Lines | ||
| Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] | ||
| Total unfavorable (favorable) prior-year development | 0.1 | 0.7 |
| Run-off Lines | ||
| Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items] | ||
| Total unfavorable (favorable) prior-year development | $ 0.1 | $ 23.4 |