Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (73.8)	$ (32.0)
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(20.0)	(18.4)
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(23.3)	$ (9.8)
Core Commercial [Member] \| Commercial Multiple Peril Line [Member] \| Catastrophe [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(17.0)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(30.5)
Personal Lines [Member] \| Homeowners [Member] \| Noncatastrophe [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(10.1)
Personal Lines [Member] \| Homeowners [Member] \| Catastrophe [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (20.2)