Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 1,311.8	$ 1,237.0	$ 3,831.0	$ 3,685.8
Net realized investment gains	13.4	4.2	21.2	5.0
Total revenues	1,325.2	1,241.2	3,852.2	3,690.8
Net investment income	76.6	67.8	220.0	205.2
Operating income (loss) before interest expense and income taxes	13.1	129.8	201.1	344.9
Interest on debt	(12.1)	(12.5)	(36.3)	(42.8)
Operating income before income taxes	1.0	117.3	164.8	302.1
Net loss from repayment of debt				(86.1)
Other non-operating items	(5.5)	2.6	(7.1)	4.1
Income before income taxes	8.9	124.1	178.9	225.1
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	648.2	628.4	1,913.2	1,857.2
Underwriting (loss) income	(13.7)	3.6	(14.1)	12.4
Net investment income	42.1	39.0	123.7	117.5
Other income (expense)	(0.2)	(0.1)	(0.8)	(0.7)
Operating income (loss) before interest expense and income taxes	28.2	42.5	108.8	129.2
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues	421.0	391.5	1,234.3	1,154.8
Underwriting (loss) income	40.8	23.3	61.9	81.2
Net realized investment gains	17.8	17.1	52.3	51.6
Other income (expense)	1.1	1.3	3.3	3.4
Operating income (loss) before interest expense and income taxes	59.7	41.7	117.5	136.2
Chaucer [Member]
Segment Reporting Information [Line Items]
Operating revenues	238.6	215.0	674.5	667.9
Underwriting (loss) income	(87.7)	37.8	(55.9)	53.8
Net investment income	13.4	10.3	37.1	32.3
Other income (expense)	0.5	0.3	(0.4)	1.5
Operating income (loss) before interest expense and income taxes	(73.8)	48.4	(19.2)	87.6
Other (including eliminations)
Segment Reporting Information [Line Items]
Operating revenues	4.0	2.1	9.0	5.9
Underwriting (loss) income	(1.0)	(0.8)	(2.8)	(2.1)
Net investment income	3.3	1.4	6.9	3.8
Other expense	(3.3)	(3.4)	(10.1)	(9.8)
Operating income (loss) before interest expense and income taxes	$ (1.0)	$ (2.8)	$ (6.0)	$ (8.1)