Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 1,245.8	$ 1,217.8
Net realized and unrealized investment gains (losses)	37.5	(161.6)
Total revenues	1,283.3	1,056.2
Net investment income	76.8	69.6
Operating income (loss) before interest expense and income taxes	85.1	117.1
Interest on debt	(8.5)	(9.4)
Operating income before income taxes	76.6	107.7
Income (loss) from continuing operations before income taxes	114.1	(53.9)
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	748.3	724.7
Underwriting income (loss)	(49.3)	10.4
Net investment income	51.8	46.4
Other income (expense)	(0.3)	(2.2)
Operating income (loss) before interest expense and income taxes	2.2	54.6
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	493.3	488.5
Underwriting income (loss)	58.9	44.1
Net investment income	22.1	20.3
Other income (expense)	0.8	0.5
Operating income (loss) before interest expense and income taxes	81.8	64.9
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	4.2	4.6
Underwriting income (loss)	(0.3)	(3.3)
Net investment income	2.9	2.9
Other income (expense)	(1.5)	(2.0)
Operating income (loss) before interest expense and income taxes	$ 1.1	$ (2.4)