Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 1,370.8	$ 1,261.2	$ 2,717.4	$ 2,505.8
Net realized and unrealized investment gains (losses)	(77.9)	31.1	(93.8)	68.6
Total revenues	1,292.9	1,292.3	2,623.6	2,574.4
Net investment income	70.5	75.6	147.4	152.4
Operating income (loss) before interest expense and income taxes	115.0	137.3	269.4	222.4
Interest on debt	(8.5)	(8.5)	(17.0)	(17.0)
Operating income before income taxes	106.5	128.8	252.4	205.4
Other non-operating	(0.4)		(0.4)
Income from continuing operations before income taxes	28.2	159.9	158.2	274.0
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	513.6	483.4	1,024.7	956.3
Underwriting (loss) income	34.3	35.1	66.5	(16.2)
Net investment income	32.6	35.0	68.0	71.9
Other income (expense)		(0.2)	(0.1)	(0.6)
Operating income (loss) before interest expense and income taxes	66.9	69.9	134.4	55.1
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	309.8	273.5	611.1	547.7
Underwriting (loss) income	30.3	18.7	64.2	20.7
Net investment income	14.8	16.0	31.0	30.9
Other income (expense)	0.1	(0.2)		(0.1)
Operating income (loss) before interest expense and income taxes	45.2	34.5	95.2	51.5
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	544.2	500.4	1,074.9	993.7
Underwriting (loss) income	(19.9)	9.5	(7.6)	68.4
Net investment income	20.6	21.8	43.2	43.9
Other income (expense)	2.1	0.9	3.5	1.7
Operating income (loss) before interest expense and income taxes	2.8	32.2	39.1	114.0
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.2	3.9	6.7	8.1
Underwriting (loss) income		(0.3)		(0.6)
Net investment income	2.5	2.8	5.2	5.7
Other income (expense)	(2.4)	(1.8)	(4.5)	(3.3)
Operating income (loss) before interest expense and income taxes	$ 0.1	$ 0.7	$ 0.7	$ 1.8