Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 1,262.3	$ 1,160.6	$ 2,508.1	$ 2,378.4
Net realized and unrealized investment gains (losses)	31.1	63.7	68.6	(97.9)
Total revenues	1,293.4	1,224.3	2,576.7	2,280.5
Net investment income	75.6	57.7	152.4	127.3
Operating income (loss) before interest expense and income taxes	137.3	88.9	222.4	206.0
Interest on debt	(8.5)	(9.4)	(17.0)	(18.8)
Operating income before income taxes	128.8	79.5	205.4	187.2
Other non-operating		(0.1)		(0.1)
Income from continuing operations before income taxes	159.9	143.1	274.0	89.2
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	758.0	698.9	1,506.3	1,423.6
Underwriting income (loss)	53.8	21.3	4.5	31.7
Net investment income	51.0	37.9	102.8	84.3
Other income (expense)	(0.4)	(3.9)	(0.7)	(6.1)
Operating income (loss) before interest expense and income taxes	104.4	55.3	106.6	109.9
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	500.4	456.3	993.7	944.8
Underwriting income (loss)	9.5	17.4	68.4	61.5
Net investment income	21.8	16.5	43.9	36.8
Other income (expense)	0.9	(1.3)	1.7	(0.8)
Operating income (loss) before interest expense and income taxes	32.2	32.6	114.0	97.5
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.9	5.4	8.1	10.0
Underwriting income (loss)	(0.3)	(0.4)	(0.6)	(3.7)
Net investment income	2.8	3.3	5.7	6.2
Other income (expense)	(1.8)	(1.9)	(3.3)	(3.9)
Operating income (loss) before interest expense and income taxes	$ 0.7	$ 1.0	$ 1.8	$ (1.4)