Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	$ 7,012.6	$ 6,447.6	$ 6,024.0	$ 5,654.4
Ceded	(1,748.6)	(1,693.8)	(1,641.6)	(1,574.8)
Net	5,264.0	4,753.8	$ 4,382.4	$ 4,079.6
Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	3,350.9	2,991.0
Ceded	(508.2)	(409.3)
Net	2,842.7	2,581.7
Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,598.1	1,522.0
Ceded	(364.0)	(388.8)
Net	1,234.1	1,133.2
Other Segments [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	65.5	66.2
Ceded	(51.4)	(52.1)
Net	14.1	14.1
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,998.1	1,868.4
Ceded	(825.0)	(843.6)
Net	1,173.1	1,024.8
Commercial Multiple Peril Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,609.1	1,378.5
Ceded	(185.2)	(153.2)
Net	1,423.9	1,225.3
Workers' Compensation Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	719.1	681.2
Ceded	(125.2)	(113.9)
Net	593.9	567.3
Commercial Automobile Line [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	469.7	467.0
Ceded	(21.8)	(23.1)
Net	447.9	443.9
General Liability and Umbrella - Occurrence [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	418.7	363.7
Ceded	(121.8)	(89.5)
Net	296.9	274.2
Other Core Commercial [Member] | Core Commercial Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	134.3	100.6
Ceded	(54.2)	(29.6)
Net	80.1	71.0
Specialty Property & Casualty [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	820.6	798.6
Ceded	(278.8)	(287.5)
Net	541.8	511.1
General Liability - Claims Made [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	381.1	345.9
Ceded	(18.7)	(18.6)
Net	362.4	327.3
Other Specialty [Member] | Specialty Lines Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	396.4	377.5
Ceded	(66.5)	(82.7)
Net	329.9	294.8
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,566.7	1,560.0
Ceded	(820.6)	(838.4)
Net	746.1	721.6
Homeowners [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	312.3	238.5
Ceded	(2.8)	(4.0)
Net	309.5	234.5
Other Personal [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	119.1	69.9
Ceded	(1.6)	(1.2)
Net	$ 117.5	$ 68.7