Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	May 21, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating revenues		$ 1,237.0	$ 1,225.5	$ 3,685.8	$ 3,799.3
Net investment income		67.8	68.3	205.2	209.1
Total revenues		1,241.2	1,233.5	3,690.8	3,829.3
Operating income (loss) before interest expense and income taxes		129.8	122.6	344.9	342.5
Interest on debt		(12.5)	(14.8)	(42.8)	(45.9)
Operating income before income taxes		117.3	107.8	302.1	296.6
Net realized investment gains		4.2	8.0	5.0	30.0
Net gain on disposal of U.K. motor business		0.1		1.3	37.7
Net loss from repayment of debt	$ (86.1)		(5.6)	(86.1)	(24.1)
Other non-operating items		2.5	0.2	2.8	(0.2)
Income before income taxes		124.1	110.4	225.1	340.0
Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues		628.4	601.2	1,857.2	1,786.7
Underwriting (loss) income		3.6	8.3	12.4	10.9
Net investment income		39.0	38.7	117.5	116.7
Other (expense) income		(0.1)	0.1	(0.7)	(1.1)
Operating income (loss) before interest expense and income taxes		42.5	47.1	129.2	126.5
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues		391.5	379.7	1,154.8	1,131.4
Underwriting (loss) income		23.3	17.9	81.2	34.3
Other (expense) income		1.3	1.3	3.4	3.1
Operating income (loss) before interest expense and income taxes		41.7	37.2	136.2	91.6
Net realized investment gains		17.1	18.0	51.6	54.2
Chaucer
Segment Reporting Information [Line Items]
Operating revenues		215.0	242.7	667.9	875.5
Underwriting (loss) income		37.8	29.6	53.8	92.7
Net investment income		10.3	10.5	32.3	34.8
Other (expense) income		0.3	1.1	1.5	4.9
Operating income (loss) before interest expense and income taxes		48.4	41.2	87.6	132.4
Other (including eliminations)
Segment Reporting Information [Line Items]
Operating revenues		2.1	1.9	5.9	5.7
Underwriting (loss) income		(0.8)	(0.9)	(2.1)	(2.3)
Net investment income		1.4	1.1	3.8	3.4
Other net expenses		(3.4)	(3.1)	(9.8)	(9.1)
Operating income (loss) before interest expense and income taxes		$ (2.8)	$ (2.9)	$ (8.1)	$ (8.0)