Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (27.2)	$ (35.8)
Workers' Compensation Line [Member] | Years 2014 To 2019 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(10.5)
Workers' Compensation Line [Member] | Years 2013 To 2017 and 2020 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(14.2)
Commercial Multiple Peril Line [Member] | Years 2013 To 2017 and 2020 [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(11.3)
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(17.5)	(7.1)
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(20.1)	(16.1)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	10.4	$ (13.2)
Personal Lines [Member] | Homeowners [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 14.1