Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 1,171.3	$ 1,114.4
Net realized and unrealized investment gains (losses)	48.2	(22.9)
Total revenues	1,219.5	1,091.5
Net investment income	70.2	66.0
Operating income (loss) before interest expense and income taxes	109.8	93.3
Interest on debt	(9.4)	(11.3)
Operating income before income taxes	100.4	82.0
Income from continuing operations before income taxes	148.6	59.1
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	698.7	676.6
Underwriting income (loss)	36.7	16.3
Net investment income	44.3	45.3
Other income (expense)	(0.8)	(0.2)
Operating income (loss) before interest expense and income taxes	80.2	61.4
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	465.2	434.9
Underwriting income (loss)	5.7	14.6
Net investment income	19.7	18.4
Other income (expense)	1.4	1.3
Operating income (loss) before interest expense and income taxes	26.8	34.3
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues	7.4	2.9
Underwriting income (loss)	(0.3)	(1.0)
Net investment income	6.2	2.3
Other expense	(3.1)	(3.7)
Operating income (loss) before interest expense and income taxes	$ 2.8	$ (2.4)