Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (4.7)	$ (27.2)
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(38.1)	(17.5)
Specialty [Member] | Professional and Executive Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(22.2)
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(1.5)	(20.1)
Core Commercial [Member] | Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(12.9)	(11.3)
Core Commercial [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(12.0)	(14.2)
Core Commercial [Member] | Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	10.5
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	34.9	10.4
Personal Lines [Member] | Homeowners [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	18.4	$ 14.1
Personal Lines [Member] | Personal Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 12.4