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	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 209.7	$ 126.0
U.S. Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	125.1	111.7
International Operations
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	77.3	11.7
Run-off Lines
Schedule Of Prior Accident Years Loss And Loss Adjustment Expense Reserves Development [Line Items]
Total unfavorable (favorable) prior-year development	$ 7.3	$ 2.6