Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	$ 7,308.1	$ 7,012.6	$ 6,447.6	$ 6,024.0
Ceded	(1,795.0)	(1,748.6)	(1,693.8)	(1,641.6)
Net	5,513.1	5,264.0	$ 4,753.8	$ 4,382.4
Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	3,468.4	3,350.9
Ceded	(571.5)	(508.2)
Net	2,896.9	2,842.7
Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,625.5	1,598.1
Ceded	(346.2)	(364.0)
Net	1,279.3	1,234.1
Other Segments [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	65.4	65.5
Ceded	(50.8)	(51.4)
Net	14.6	14.1
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	2,148.8	1,998.1
Ceded	(826.5)	(825.0)
Net	1,322.3	1,173.1
Commercial Multiple Peril Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,620.3	1,609.1
Ceded	(200.8)	(185.2)
Net	1,419.5	1,423.9
Workers' Compensation Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	724.1	719.1
Ceded	(126.5)	(125.2)
Net	597.6	593.9
Commercial Automobile Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	491.2	469.7
Ceded	(23.9)	(21.8)
Net	467.3	447.9
General Liability and Umbrella - Occurrence [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	519.2	418.7
Ceded	(187.0)	(121.8)
Net	332.2	296.9
Other Core Commercial [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	113.6	134.3
Ceded	(33.3)	(54.2)
Net	80.3	80.1
Specialty Property & Casualty [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	861.6	820.6
Ceded	(296.2)	(278.8)
Net	565.4	541.8
General Liability - Claims Made [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	406.9	381.1
Ceded	(22.4)	(18.7)
Net	384.5	362.4
Other Specialty [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	357.0	396.4
Ceded	(27.6)	(66.5)
Net	329.4	329.9
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,621.9	1,566.7
Ceded	(822.4)	(820.6)
Net	799.5	746.1
Homeowners [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	377.4	312.3
Ceded	(1.7)	(2.8)
Net	375.7	309.5
Other Personal [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	149.5	119.1
Ceded	(2.4)	(1.6)
Net	$ 147.1	$ 117.5