Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 6,026.0	$ 5,575.1	$ 5,104.8
Net realized and unrealized investment gains (losses)	(32.5)	(106.5)	123.0
Total revenues	5,993.5	5,468.6	5,227.8
Net investment income	332.1	296.3	310.7
Operating income (loss) before interest expense and income taxes	105.6	285.1	432.3
Interest on debt	(34.1)	(34.1)	(34.0)
Operating income before income taxes	71.5	251.0	398.3
Other non-operating items	2.1	(0.5)
Income from continuing operations before income taxes	41.1	144.0	521.3
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,216.8	2,090.7	1,960.7
Underwriting (loss) income	16.6	(28.7)	(7.5)
Net investment income	151.8	136.2	146.5
Other income (expense)	(1.2)	(0.6)	(1.0)
Operating income (loss) before interest expense and income taxes	167.2	106.9	138.0
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,351.7	1,256.5	1,099.2
Underwriting (loss) income	172.9	124.7	69.4
Net investment income	71.1	62.1	62.9
Other income (expense)	(0.5)	(0.8)	(0.4)
Operating income (loss) before interest expense and income taxes	243.5	186.0	131.9
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,442.1	2,213.7	2,028.5
Underwriting (loss) income	(411.6)	(104.2)	65.1
Net investment income	96.8	86.8	89.4
Other income (expense)	10.5	8.6	4.0
Operating income (loss) before interest expense and income taxes	(304.3)	(8.8)	158.5
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	15.4	14.2	16.4
Underwriting (loss) income	(2.3)	(1.2)	(1.0)
Net investment income	12.4	11.2	11.9
Other income (expense)	(10.9)	(9.0)	(7.0)
Operating income (loss) before interest expense and income taxes	$ (0.8)	$ 1.0	$ 3.9