Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 1,143.6	$ 1,072.6	$ 3,387.7	$ 3,154.9
Net realized and unrealized investment gains	23.0	13.3	3.8	20.5
Total revenues	1,166.6	1,085.9	3,391.5	3,175.4
Net investment income	66.4	62.6	198.0	181.3
Operating income (loss) before interest expense and income taxes	112.8	86.2	313.4	218.3
Interest on debt	(11.2)	(11.3)	(33.9)	(33.9)
Operating income before income taxes	101.6	74.9	279.5	184.4
Other non-operating items	(1.9)	(5.5)	(1.9)	(7.3)
Income from continuing operations before income taxes	122.7	82.7	281.4	197.6
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	687.4	648.2	2,045.7	1,913.2
Underwriting income (loss)	20.8	(13.9)	74.6	(14.7)
Net investment income	45.0	42.1	135.1	123.7
Other income (expense)	(0.7)	(0.2)	(1.4)	(0.8)
Operating income (loss) before interest expense and income taxes	65.1	28.0	208.3	108.2
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	452.4	421.0	1,331.8	1,234.3
Underwriting income (loss)	28.8	40.7	51.3	61.5
Net investment income	18.2	17.8	54.7	52.3
Other income (expense)	1.2	1.1	3.7	3.3
Operating income (loss) before interest expense and income taxes	48.2	59.6	109.7	117.1
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.8	3.4	10.2	7.4
Underwriting income (loss)	(1.0)	(0.8)	(3.1)	(2.2)
Net investment income	3.2	2.7	8.2	5.3
Other expense	(2.7)	(3.3)	(9.7)	(10.1)
Operating income (loss) before interest expense and income taxes	$ (0.5)	$ (1.4)	$ (4.6)	$ (7.0)