Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (31.2)	$ (56.7)
Workers' Compensation Line [Member] | Years 2014 To 2019 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(12.8)
Workers' Compensation Line [Member] | Years 2013 To 2018 and 2020 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(21.0)
Commercial Multiple Peril Line [Member] | Years 2014 To 2019 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(11.8)
Commercial Multiple Peril Line [Member] | Years 2013 To 2018 and 2020 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(9.9)
Commercial Automobile Line [Member] | Years 2013 To 2018 and 2020 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(11.5)
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(24.2)	(14.3)
Specialty [Member] | Professional and Executive Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(13.2)
Specialty [Member] | Surety Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(11.5)
Specialty [Member] | Marine Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(10.2)	(13.8)
Specialty [Member] | Specialty P&C Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(10.5)
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(20.2)	(20.3)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	13.2	(23.1)
Personal Lines [Member] | Homeowners [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 19.4
Personal Lines [Member] | Personal Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		$ (20.1)