Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues									$ 4,545.0	$ 4,246.8	$ 4,043.7
Net realized and unrealized investment gains (losses)									(50.7)	21.1	10.2
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3	4,267.9	4,053.9
Net investment income									267.4	243.9	231.6
Operating income (loss) before interest expense and income taxes									406.5	327.3	192.9
Interest on debt									(45.1)	(45.2)	(51.4)
Operating income before income taxes									361.4	282.1	141.5
Net loss from repayment of debt									(28.2)		(88.3)
Other non-operating items										(10.3)	2.6
Income from continuing operations before income taxes									282.5	292.9	66.0
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,739.5	2,573.8	2,485.0
Underwriting income (loss)									85.0	12.2	(123.2)
Net investment income									182.2	165.8	158.5
Other income (expense)									(1.5)	(0.6)	(0.6)
Operating income (loss) before interest expense and income taxes									265.7	177.4	34.7
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,791.3	1,662.3	1,552.4
Underwriting income (loss)									66.6	83.7	103.1
Net investment income									73.7	70.1	69.5
Other income (expense)									5.9	4.9	5.1
Operating income (loss) before interest expense and income taxes									146.2	158.7	177.7
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues									14.2	10.7	6.3
Underwriting income (loss)									(4.2)	(4.2)	(9.9)
Net investment income									11.5	8.0	3.6
Other expense									(12.7)	(12.6)	(13.2)
Operating income (loss) before interest expense and income taxes									$ (5.4)	$ (8.8)	$ (19.5)