Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 5,575.1	$ 5,104.8	$ 4,819.8
Net realized and unrealized investment gains (losses)	(106.5)	123.0	5.0
Total revenues	5,468.6	5,227.8	4,824.8
Net investment income	296.3	310.7	265.1
Operating income (loss) before interest expense and income taxes	285.1	432.3	484.7
Interest on debt	(34.1)	(34.0)	(37.1)
Operating income before income taxes	251.0	398.3	447.6
Net loss from repayment of debt			(6.2)
Other non-operating items	(0.5)		(1.6)
Income from continuing operations before income taxes	144.0	521.3	444.8
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,090.7	1,960.7	1,832.2
Underwriting (loss) income	(28.7)	(7.5)	57.5
Net investment income	136.2	146.5	124.2
Other income (expense)	(0.6)	(1.0)	(3.6)
Operating income (loss) before interest expense and income taxes	106.9	138.0	178.1
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,256.5	1,099.2	1,036.1
Underwriting (loss) income	124.7	69.4	48.8
Net investment income	62.1	62.9	51.1
Other income (expense)	(0.8)	(0.4)	(2.6)
Operating income (loss) before interest expense and income taxes	186.0	131.9	97.3
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	2,213.7	2,028.5	1,931.4
Underwriting (loss) income	(104.2)	65.1	132.9
Net investment income	86.8	89.4	76.7
Other income (expense)	8.6	4.0	2.9
Operating income (loss) before interest expense and income taxes	(8.8)	158.5	212.5
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	14.2	16.4	20.1
Underwriting (loss) income	(1.2)	(1.0)	(4.3)
Net investment income	11.2	11.9	13.1
Other income (expense)	(9.0)	(7.0)	(12.0)
Operating income (loss) before interest expense and income taxes	$ 1.0	$ 3.9	$ (3.2)