Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability For Claims And Claims Adjustment Expense [Line Items]
Catastrophe losses	$ 690.1	$ 402.6	$ 402.6
(Favorable) unfavorable loss and LAE development	(15.9)	(32.6)	(71.1)
Loss and loss adjustment expense reserves	$ 7,308.1	7,012.6	6,447.6	$ 6,024.0
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	$ 57.5	57.1
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	40.9	22.0	(26.1)
Loss and loss adjustment expense reserves	2,148.8	1,998.1
Other Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	2.3	1.2	1.0
Loss and loss adjustment expense reserves	65.4	65.5
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(2.0)	(27.6)	(27.5)
Loss and loss adjustment expense reserves	3,468.4	3,350.9
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(57.1)	(28.2)	(18.5)
Loss and loss adjustment expense reserves	1,625.5	1,598.1
Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	35.4
Workers' Compensation Line [Member] | Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	724.1	719.1
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	14.2		24.8
Loss and loss adjustment expense reserves	1,621.9	1,566.7
Professional and Executive Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(36.5)	(26.3)
Surety Lines [Member] | Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	12.0	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	861.6	820.6
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	1,620.3	1,609.1
Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	15.5
Commercial Automobile Line [Member] | Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	491.2	469.7
Homeowners Line Segment [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	11.5	29.3
Standalone Personal Umbrella [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	13.4
Commercial Umbrella [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ 10.4
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 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 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