Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 1,411.2	$ 1,270.9	$ 4,128.6	$ 3,776.7
Net realized and unrealized investment gains (losses)	(44.9)	4.0	(138.7)	72.6
Total revenues	1,366.3	1,274.9	3,989.9	3,849.3
Net investment income	73.0	78.8	220.4	231.2
Operating income (loss) before interest expense and income taxes	53.9	47.0	323.3	269.4
Interest on debt	(8.5)	(8.5)	(25.5)	(25.5)
Operating income before income taxes	45.4	38.5	297.8	243.9
Other non-operating			(0.4)
Income from continuing operations before income taxes	0.5	42.5	158.7	316.5
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	527.1	497.6	1,551.8	1,453.9
Underwriting (loss) income	(7.7)	(25.7)	58.8	(41.9)
Net investment income	33.4	37.0	101.4	108.9
Other income (expense)	(0.5)	(0.3)	(0.6)	(0.9)
Operating income (loss) before interest expense and income taxes	25.2	11.0	159.6	66.1
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	319.9	256.4	931.0	804.1
Underwriting (loss) income	32.0	15.5	96.2	36.2
Net investment income	15.2	15.9	46.2	46.8
Other income (expense)	(0.3)		(0.3)	(0.1)
Operating income (loss) before interest expense and income taxes	46.9	31.4	142.1	82.9
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	560.5	512.6	1,635.4	1,506.3
Underwriting (loss) income	(42.7)	(20.3)	(50.3)	48.1
Net investment income	21.4	22.6	64.6	66.5
Other income (expense)	2.5	1.3	6.0	3.0
Operating income (loss) before interest expense and income taxes	(18.8)	3.6	20.3	117.6
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.7	4.3	10.4	12.4
Underwriting (loss) income		(0.4)		(1.0)
Net investment income	3.0	3.3	8.2	9.0
Other income (expense)	(2.4)	(1.9)	(6.9)	(5.2)
Operating income (loss) before interest expense and income taxes	$ 0.6	$ 1.0	$ 1.3	$ 2.8