Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	May 21, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Operating revenues		$ 1,222.7	$ 1,284.5	$ 2,448.8	$ 2,573.8
Net investment income		69.1	70.7	137.4	140.8
Total revenues		1,222.0	1,297.1	2,449.6	2,595.8
Operating income (loss) before interest expense and income taxes		94.7	119.5	215.1	219.9
Interest on debt		(15.6)	(15.0)	(30.3)	(31.1)
Operating income before income taxes		79.1	104.5	184.8	188.8
Net realized investment (losses) gains		(0.7)	12.6	0.8	22.0
Net gain on disposal of U.K. motor business		0.4	37.7	1.2	37.7
Net loss from repayment of debt	$ (86.1)	(86.1)	(1.8)	(86.1)	(18.5)
Other non-operating items		0.2	(0.4)	0.3	(0.4)
Income (loss) before income taxes		(7.1)	152.6	101.0	229.6
Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues		616.1	598.8	1,228.8	1,185.5
Underwriting (loss) income		5.1	8.5	8.8	2.6
Net investment income		39.1	39.4	78.5	78.0
Other (expense) income		(0.2)	0.3	(0.6)	(1.2)
Operating income (loss) before interest expense and income taxes		44.0	48.2	86.7	79.4
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues		384.5	377.7	763.3	751.7
Underwriting (loss) income		29.3	13.0	57.9	16.4
Other (expense) income		1.0	0.8	2.1	1.8
Operating income (loss) before interest expense and income taxes		47.4	32.1	94.5	54.4
Net realized investment (losses) gains		17.1	18.3	34.5	36.2
Chaucer
Segment Reporting Information [Line Items]
Operating revenues		219.8	306.3	452.9	632.8
Underwriting (loss) income		(6.5)	27.5	16.0	63.1
Net investment income		11.3	12.0	22.0	24.3
Other (expense) income		0.7	2.5	1.2	3.8
Operating income (loss) before interest expense and income taxes		5.5	42.0	39.2	91.2
Other (including eliminations)
Segment Reporting Information [Line Items]
Operating revenues		2.3	1.7	3.8	3.8
Underwriting (loss) income		(0.6)	(0.7)	(1.3)	(1.4)
Net investment income		1.6	1.0	2.4	2.3
Other net expenses		(3.2)	(3.1)	(6.4)	(6.0)
Operating income (loss) before interest expense and income taxes		$ (2.2)	$ (2.8)	$ (5.3)	$ (5.1)