Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability For Claims And Claims Adjustment Expense [Line Items]
Catastrophe losses	$ 402.6	$ 402.6	$ 286.7
(Favorable) unfavorable loss and LAE development	(32.6)	(71.1)	(32.6)
Loss and loss adjustment expense reserves	$ 7,012.6	6,447.6	6,024.0	$ 5,654.4
U.S. Companies [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross incurred claims, percentage	4.00%
Asbestos Issue [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 57.1	57.3
Winter Storm Elliott [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Catastrophe losses	165.0
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	22.0	(26.1)	1.0	$ (26.1)
Loss and loss adjustment expense reserves	1,998.1	1,868.4
Other Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	1.2	1.0	4.2
Loss and loss adjustment expense reserves	65.5	66.2
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(27.6)	(27.5)	(27.3)
Loss and loss adjustment expense reserves	3,350.9	2,991.0
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(28.2)	(18.5)	(10.5)
Loss and loss adjustment expense reserves	1,598.1	1,522.0
Workers' Compensation Line [Member] | Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	719.1	681.2
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		24.8
Loss and loss adjustment expense reserves	1,566.7	1,560.0
Professional and Executive Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	26.3
Surety Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	14.5
Marine Line [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	10.9
Specialty Property & Casualty [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	23.5
Loss and loss adjustment expense reserves	820.6	798.6
Specialty Property & Casualty [Member] | Specialty Industrial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	31.0
Commercial Multiple Peril Line [Member] | Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	1,609.1	1,378.5
Commercial Automobile Line [Member] | Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	469.7	467.0
Homeowners Line Segment [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	29.3
Years 2014 to 2020 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		22.3
Years 2014 to 2020 [Member] | Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		12.3
Years 2016 to 2019 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			34.9
Years 2016 to 2019 [Member] | Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			15.6
Years 2013 to 2018 and 2020 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	32.1
Years 2013 to 2018 and 2020 [Member] | Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	16.4
Years 2013 to 2018 and 2020 [Member] | Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 18.0
Years 2017 to 2019 [Member] | Professional and Executive Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			10.1
Years 2017 to 2019 [Member] | Marine Line [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			$ 20.6
Years 2019 to 2020 [Member] | Marine Line [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		14.4
Years 2013 to 2016, 2018 and 2019 [Member] | Surety Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		10.0
Years 2013 to 2016, 2018 and 2019 [Member] | Specialty Property & Casualty [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		$ 12.0