Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues									$ 5,014.5	$ 5,017.5	$ 4,760.2
Net realized investment gains									19.5	50.1	33.5
Total revenues	$ 1,204.7	$ 1,233.5	$ 1,297.1	$ 1,298.7	$ 1,285.1	$ 1,265.6	$ 1,273.0	$ 1,243.9	5,034.0	5,067.6	4,793.7
Net investment income									279.1	270.3	269.0
Interest on debt									(60.1)	(65.2)	(65.3)
Operating income before income taxes									405.5	341.0	328.1
Gain on disposal of U.K. motor business									38.4
Loss from settlement of pension obligation										(12.1)
Net loss from repayment of debt									(24.1)	(0.1)	(27.7)
Other non-operating items									0.1	(0.9)	(4.8)
Income before income taxes									439.4	378.0	329.1
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating income (loss) before interest expense and income taxes									465.6	406.2	393.4
Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues									2,391.7	2,239.0	2,109.5
Underwriting income (loss)									(12.7)	(8.3)	(11.0)
Net investment income									156.3	149.4	143.5
Other income (expense)									(0.3)	(1.2)	(0.1)
Operating income (loss) before interest expense and income taxes									143.3	139.9	132.4
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues									1,511.3	1,491.0	1,542.5
Underwriting income (loss)									72.0	22.1	37.6
Net investment income									72.5	71.9	75.8
Other income (expense)									4.8	5.0	5.2
Operating income (loss) before interest expense and income taxes									149.3	99.0	118.6
Chaucer
Segment Reporting Information [Line Items]
Operating revenues									1,104.1	1,279.7	1,098.2
Underwriting income (loss)									131.5	122.5	107.7
Net investment income									45.9	44.2	42.7
Other income (expense)									6.3	10.9
Operating income (loss) before interest expense and income taxes									183.7	177.6	150.4
Other
Segment Reporting Information [Line Items]
Operating revenues									7.4	7.8	10.0
Underwriting income (loss)									(1.9)	(3.1)	(3.4)
Net investment income									4.4	4.8	7.0
Other net expenses									(13.2)	(12.0)	(11.6)
Operating income (loss) before interest expense and income taxes									$ (10.7)	$ (10.3)	$ (8.0)