Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 1,272.4	$ 1,212.1	$ 3,780.5	$ 3,590.5
Net realized and unrealized investment gains (losses)	4.0	37.7	72.6	(60.2)
Total revenues	1,276.4	1,249.8	3,853.1	3,530.3
Net investment income	78.8	67.6	231.2	194.9
Operating income (loss) before interest expense and income taxes	47.0	128.4	269.4	334.4
Interest on debt	(8.5)	(9.8)	(25.5)	(28.6)
Operating income before income taxes	38.5	118.6	243.9	305.8
Loss from repayment of debt		(6.1)		(6.2)
Other non-operating		(1.6)		(1.6)
Income from continuing operations before income taxes	42.5	148.6	316.5	237.8
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	755.5	713.3	2,261.8	2,136.9
Underwriting (loss) income	(10.2)	18.7	(5.7)	50.4
Net investment income	52.9	44.4	155.7	128.7
Other income (expense)	(0.3)	(0.8)	(1.0)	(6.9)
Operating income (loss) before interest expense and income taxes	42.4	62.3	149.0	172.2
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	512.6	493.3	1,506.3	1,438.1
Underwriting (loss) income	(20.3)	43.0	48.1	104.5
Net investment income	22.6	19.5	66.5	56.3
Other income (expense)	1.3	1.7	3.0	0.9
Operating income (loss) before interest expense and income taxes	3.6	64.2	117.6	161.7
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	4.3	5.5	12.4	15.5
Underwriting (loss) income	(0.4)	(0.3)	(1.0)	(4.0)
Net investment income	3.3	3.7	9.0	9.9
Other income (expense)	(1.9)	(1.5)	(5.2)	(5.4)
Operating income (loss) before interest expense and income taxes	$ 1.0	$ 1.9	$ 2.8	$ 0.5